diff --git a/app-desktop/build.gradle.kts b/app-desktop/build.gradle.kts
index 5d4bd25c..ba0a336b 100644
--- a/app-desktop/build.gradle.kts
+++ b/app-desktop/build.gradle.kts
@@ -66,11 +66,14 @@ val generateStrings by tasks.registering(JavaExec::class) {
     mainClass.set("kst4contest.i18n.GeneratorMain")
 
     val input = layout.projectDirectory.dir("src/main/i18n")
+    // A local copy: the provider below must not capture the script-level val, which
+    // drags the build script object into the configuration cache and fails to serialize.
+    val output = generatedI18nDirectory
     inputs.dir(input)
-    outputs.dir(generatedI18nDirectory)
+    outputs.dir(output)
 
     argumentProviders.add(CommandLineArgumentProvider {
-        listOf(input.asFile.absolutePath, generatedI18nDirectory.get().asFile.absolutePath)
+        listOf(input.asFile.absolutePath, output.get().asFile.absolutePath)
     })
 }
 
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/DrawingLayerRegressionTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/DrawingLayerRegressionTest.kt
index 3c7bd4c1..96ae82af 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/DrawingLayerRegressionTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/DrawingLayerRegressionTest.kt
@@ -20,6 +20,8 @@ import kst4contest.view.compose.tabs.MessageHandlingTabState
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
 /**
  * The four ways the drawing layer went wrong while the build was green.
@@ -41,7 +43,7 @@ import org.junit.jupiter.api.Test
  * these were written, because the spec left it open and a net that only works on a
  * developer's desk is worth saying so about.
  */
-class DrawingLayerRegressionTest {
+class DrawingLayerRegressionTest(@TempDir private val tempDir: Path) {
 
     private fun themed(content: @Composable () -> Unit): @Composable () -> Unit = {
         Kst4ContestTheme(darkMode = false, baseFontSizeSp = 12f) { content() }
@@ -52,7 +54,7 @@ class DrawingLayerRegressionTest {
     @OptIn(ExperimentalTestApi::class)
     @Test
     fun aTypedSettingAppearsOnScreenAndNotOnlyInThePreferences() = runComposeUiTest {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.setMessageHandling_autoAnswerTextMainCat("")
         val state = MessageHandlingTabState(prefs)
 
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/MainWindowStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/MainWindowStateTest.kt
index f1288a96..5666414e 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/MainWindowStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/MainWindowStateTest.kt
@@ -6,6 +6,8 @@ import kst4contest.model.ChatPreferences
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
 /**
  * The shortcut and snippet rosters are ChatPreferences.SimpleRoster, which notifies
@@ -13,11 +15,11 @@ import org.junit.jupiter.api.Test
  * above the send line follow the settings window — the reason ShortcutsTabState has been
  * calling refreshShortcutButtons into thin air since Etappe 3b.
  */
-class MainWindowStateRosterTest {
+class MainWindowStateRosterTest(@TempDir private val tempDir: Path) {
 
     @Test
     fun `the shortcuts start out as what the profile holds`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.getLst_txtShortCutBtnList().setAll(listOf("MYQRG", "pse sked"))
 
         val mirror = RosterMirror(prefs.getLst_txtShortCutBtnList())
@@ -31,7 +33,7 @@ class MainWindowStateRosterTest {
      */
     @Test
     fun `a roster change reaches the mirror when it is refreshed`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.getLst_txtShortCutBtnList().setAll(listOf("MYQRG"))
         val mirror = RosterMirror(prefs.getLst_txtShortCutBtnList())
 
@@ -44,7 +46,7 @@ class MainWindowStateRosterTest {
 
     @Test
     fun `an emptied roster empties the mirror`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.getLst_txtShortCutBtnList().setAll(listOf("MYQRG"))
         val mirror = RosterMirror(prefs.getLst_txtShortCutBtnList())
 
@@ -60,7 +62,7 @@ class MainWindowStateRosterTest {
      */
     @Test
     fun `drawing the buttons subscribes to the mirror`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.getLst_txtShortCutBtnList().setAll(listOf("MYQRG"))
         val mirror = RosterMirror(prefs.getLst_txtShortCutBtnList())
 
@@ -79,9 +81,9 @@ class MainWindowStateRosterTest {
  * A mismatch here is silent and total: SplitterState falls back to an even split, so
  * every operator would lose the layout they arranged without any error saying so.
  */
-class MainWindowSplitterDefaultsTest {
+class MainWindowSplitterDefaultsTest(@TempDir private val tempDir: Path) {
 
-    private val prefs = ChatPreferences()
+    private val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
 
     @Test
     fun `the message column has as many dividers as the profile stores positions`() {
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/ObservedValueTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/ObservedValueTest.kt
index 97e74adc..d0a1966e 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/ObservedValueTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/ObservedValueTest.kt
@@ -5,6 +5,8 @@ import kst4contest.model.ChatPreferences
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
 /**
  * ChatPreferences hands out MutableValue, which notifies its own listeners and nothing
@@ -15,11 +17,11 @@ import org.junit.jupiter.api.Test
  * The JavaFX window solved it with attachQtfFollower() and attachOwnQrgFollower(); this is
  * the same idea, ending in Compose state instead of a TextField.
  */
-class ObservedValueTest {
+class ObservedValueTest(@TempDir private val tempDir: Path) {
 
     @Test
     fun `the mirror starts at the stored value`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.getMYQRGFirstCat().set("144.370")
 
         val observed = ObservedValue(prefs.getMYQRGFirstCat())
@@ -30,7 +32,7 @@ class ObservedValueTest {
     /** The rotator moving must reach the screen without anything asking it to. */
     @Test
     fun `a change to the underlying value reaches the mirror by itself`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.getActualQTF().set(90.0)
         val observed = ObservedValue(prefs.getActualQTF())
 
@@ -41,7 +43,7 @@ class ObservedValueTest {
 
     @Test
     fun `a null value is carried through rather than turned into a default`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val observed = ObservedValue(prefs.getMYQRGSecondCat())
 
         prefs.getMYQRGSecondCat().set(null)
@@ -55,7 +57,7 @@ class ObservedValueTest {
      */
     @Test
     fun `drawing from the mirror subscribes to it`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.getMYQRGFirstCat().set("144.370")
         val observed = ObservedValue(prefs.getMYQRGFirstCat())
 
@@ -73,7 +75,7 @@ class ObservedValueTest {
      */
     @Test
     fun `releasing the mirror stops it following`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.getMYQRGFirstCat().set("144.370")
         val observed = ObservedValue(prefs.getMYQRGFirstCat())
 
@@ -85,7 +87,7 @@ class ObservedValueTest {
 
     @Test
     fun `releasing twice is harmless`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val observed = ObservedValue(prefs.getMYQRGFirstCat())
         observed.release()
         observed.release()
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/PaletteStoreFactoryTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/PaletteStoreFactoryTest.kt
index 4ff99d7e..58bd0230 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/PaletteStoreFactoryTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/PaletteStoreFactoryTest.kt
@@ -7,6 +7,8 @@ import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.Test
 import java.io.File
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
 /**
  * The factory binds the store to one profile's preferences and one profile's stylesheet.
@@ -15,7 +17,7 @@ import java.io.File
  * another's colours. The stylesheet path is injected here rather than derived inside, so the
  * test does not have to write into the operator's home directory to check which file is read.
  */
-class PaletteStoreFactoryTest {
+class PaletteStoreFactoryTest(@TempDir private val tempDir: Path) {
 
     private val daylight = OperatorProfilePaletteFiles(
         dayFile = File("/nonexistent/day.css"),
@@ -24,7 +26,7 @@ class PaletteStoreFactoryTest {
 
     @Test
     fun aChangeLandsInTheDesignsOwnPreference() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
 
         val store = PaletteStoreFactory.create(prefs, daylight)
         store.set(darkMode = false, role = PaletteRole.ACCENT, colour = "#FF0000")
@@ -35,7 +37,7 @@ class PaletteStoreFactoryTest {
 
     @Test
     fun anEveningChangeLandsInTheEveningPreference() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
 
         val store = PaletteStoreFactory.create(prefs, daylight)
         store.set(darkMode = true, role = PaletteRole.ACCENT, colour = "#FF0000")
@@ -47,7 +49,7 @@ class PaletteStoreFactoryTest {
     @Test
     fun whatIsAlreadyStoredIsInForceFromTheStart() {
         // The store is built at startup; a colour saved last session has to be there at once.
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.guiOptions_paletteOverridesDay = "accent=#FF0000"
 
         val store = PaletteStoreFactory.create(prefs, daylight)
@@ -57,7 +59,7 @@ class PaletteStoreFactoryTest {
 
     @Test
     fun aMissingStylesheetLeavesEveryRoleWithTheShippedSheet() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
 
         val store = PaletteStoreFactory.create(prefs, daylight)
 
@@ -73,7 +75,7 @@ class PaletteStoreFactoryTest {
             file.writeText(".root { -fx-accent: #123456; }")
 
             val store = PaletteStoreFactory.create(
-                ChatPreferences(),
+                ChatPreferences(tempDir.resolve("preferences.xml")),
                 OperatorProfilePaletteFiles(dayFile = file, eveningFile = file),
             )
 
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/AirscoutTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/AirscoutTabStateTest.kt
index 5a9c8e5e..9010ffba 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/AirscoutTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/AirscoutTabStateTest.kt
@@ -5,11 +5,13 @@ import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertFalse
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class AirscoutTabStateTest {
+class AirscoutTabStateTest(@TempDir private val tempDir: Path) {
 
-    private class Session {
-        val prefs = ChatPreferences()
+    private inner class Session {
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val state by lazy { AirscoutTabState(prefs) }
     }
 
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/BeaconTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/BeaconTabStateTest.kt
index c531ab33..88725471 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/BeaconTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/BeaconTabStateTest.kt
@@ -8,11 +8,13 @@ import org.junit.jupiter.api.Assertions.assertFalse
 import org.junit.jupiter.api.Assertions.assertNotNull
 import org.junit.jupiter.api.Assertions.assertNull
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class BeaconTabStateTest {
+class BeaconTabStateTest(@TempDir private val tempDir: Path) {
 
-    private class Session(private val templateRefusal: String? = null) {
-        val prefs = ChatPreferences()
+    private inner class Session(private val templateRefusal: String? = null) {
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         var timerRestarts = 0
         var validated = mutableListOf<String>()
 
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/GuiOptionsTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/GuiOptionsTabStateTest.kt
index 9e146fdd..30933653 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/GuiOptionsTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/GuiOptionsTabStateTest.kt
@@ -9,8 +9,10 @@ import java.util.Locale
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertFalse
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class GuiOptionsTabStateTest {
+class GuiOptionsTabStateTest(@TempDir private val tempDir: Path) {
 
     /*
      * Over lambdas rather than over the preferences, so switching the language in a test does
@@ -33,7 +35,7 @@ class GuiOptionsTabStateTest {
          * that gets saved, and it has to do so through the store, which is what repaints the
          * open windows. A store wired to anything else would pass a weaker test.
          */
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val state = GuiOptionsTabState(prefs, LanguageStoreFactory.create(prefs, Locale.UK))
 
         state.language = "de"
@@ -45,7 +47,7 @@ class GuiOptionsTabStateTest {
 
     @Test
     fun `reading comes straight from the preferences`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.setGUI_darkModeActiveByDefault(true)
 
         assertTrue(GuiOptionsTabState(prefs, languageStore()).gUI_darkModeActiveByDefault)
@@ -53,7 +55,7 @@ class GuiOptionsTabStateTest {
 
     @Test
     fun `writing reaches the preferences at once`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val state = GuiOptionsTabState(prefs, languageStore())
 
         state.guiOptions_defaultFilterPublicMsgs = true
@@ -68,7 +70,7 @@ class GuiOptionsTabStateTest {
         // The JavaFX tab offered them as four separate checkboxes, not a choice of
         // one. Turning them into a radio group would take a combination away that
         // an operator may have configured.
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val state = GuiOptionsTabState(prefs, languageStore())
 
         state.guiOptions_defaultFilterPublicMsgs = true
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/LogSynchTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/LogSynchTabStateTest.kt
index 7e5afc61..cff3a617 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/LogSynchTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/LogSynchTabStateTest.kt
@@ -5,11 +5,13 @@ import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertFalse
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class LogSynchTabStateTest {
+class LogSynchTabStateTest(@TempDir private val tempDir: Path) {
 
-    private class Session(detected: String? = null) {
-        val prefs = ChatPreferences()
+    private inner class Session(detected: String? = null) {
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         var restarts = 0
         var stops = 0
         val detectedBroadcast = detected
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/MessageHandlingTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/MessageHandlingTabStateTest.kt
index 4e6ea1a6..45682fbb 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/MessageHandlingTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/MessageHandlingTabStateTest.kt
@@ -5,12 +5,14 @@ import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertFalse
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class MessageHandlingTabStateTest {
+class MessageHandlingTabStateTest(@TempDir private val tempDir: Path) {
 
     @Test
     fun `the state starts from the stored preferences`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         prefs.setMessageHandling_autoAnswerEnabled(true)
         prefs.setMessageHandling_autoAnswerToQRGRequestEnabled(true)
         prefs.setMessageHandling_autoAnswerTextMainCat("qrv 144.300")
@@ -26,7 +28,7 @@ class MessageHandlingTabStateTest {
     fun `one checkbox drives both category flags`() {
         // The JavaFX window offered a single checkbox and wrote both preferences from
         // it; keeping only the main one would leave the second chat on its old value.
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val state = MessageHandlingTabState(prefs)
 
         state.autoAnswerEnabled = true
@@ -42,7 +44,7 @@ class MessageHandlingTabStateTest {
 
     @Test
     fun `one text field drives both category texts`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val state = MessageHandlingTabState(prefs)
 
         state.autoAnswerText = "qrv 432.200"
@@ -53,7 +55,7 @@ class MessageHandlingTabStateTest {
 
     @Test
     fun `the QRG reply flag writes through on its own`() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val state = MessageHandlingTabState(prefs)
 
         state.autoAnswerToQRGRequestEnabled = true
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/NotificationTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/NotificationTabStateTest.kt
index 4b561158..f8e4a7c0 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/NotificationTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/NotificationTabStateTest.kt
@@ -11,11 +11,13 @@ import org.junit.jupiter.api.Assertions.assertNotNull
 import org.junit.jupiter.api.Assertions.assertNull
 import org.junit.jupiter.api.Assertions.assertSame
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class NotificationTabStateTest {
+class NotificationTabStateTest(@TempDir private val tempDir: Path) {
 
-    private class Session {
-        val prefs = ChatPreferences()
+    private inner class Session {
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val monitored = SimpleRoster<String>()
         var serverEnabledApplied: Boolean? = null
         var portApplied: Int? = null
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/ShortcutsTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/ShortcutsTabStateTest.kt
index ac19a9e3..b0621482 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/ShortcutsTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/ShortcutsTabStateTest.kt
@@ -5,11 +5,13 @@ import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertSame
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class ShortcutsTabStateTest {
+class ShortcutsTabStateTest(@TempDir private val tempDir: Path) {
 
-    private class Session {
-        val prefs = ChatPreferences()
+    private inner class Session {
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         var shortcutRefreshes = 0
         var snippetRefreshes = 0
 
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/StationTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/StationTabStateTest.kt
index 80f0cf86..169db403 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/StationTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/StationTabStateTest.kt
@@ -6,11 +6,13 @@ import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertNull
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class StationTabStateTest {
+class StationTabStateTest(@TempDir private val tempDir: Path) {
 
-    private class Session {
-        val prefs = ChatPreferences()
+    private inner class Session {
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         var mainApplied: ChatCategory? = null
         var secondApplied: ChatCategory? = null
         var secondWasApplied = false
@@ -87,7 +89,7 @@ class StationTabStateTest {
     fun `the second chat switch lives with the login settings`() {
         // It moved here from the GUI tab: the JavaFX checkbox stood on the Station tab
         // next to the category pickers, and it decides what the Connect button does.
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val state = StationTabState(prefs, { }, { })
 
         state.loginToSecondChatEnabled = true
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/TrxSynchTabStateTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/TrxSynchTabStateTest.kt
index a70cb9ca..f42d11b0 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/TrxSynchTabStateTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/compose/tabs/TrxSynchTabStateTest.kt
@@ -5,11 +5,13 @@ import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertFalse
 import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
-class TrxSynchTabStateTest {
+class TrxSynchTabStateTest(@TempDir private val tempDir: Path) {
 
-    private class Session {
-        val prefs = ChatPreferences()
+    private inner class Session {
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
         val followerCalls = mutableListOf<Boolean>()
         val state by lazy { TrxSynchTabState(prefs) { followerCalls.add(it) } }
     }
diff --git a/app-desktop/src/test/kotlin/kst4contest/view/i18n/TranslationBoundaryTest.kt b/app-desktop/src/test/kotlin/kst4contest/view/i18n/TranslationBoundaryTest.kt
index b134bfef..1e8dbae8 100644
--- a/app-desktop/src/test/kotlin/kst4contest/view/i18n/TranslationBoundaryTest.kt
+++ b/app-desktop/src/test/kotlin/kst4contest/view/i18n/TranslationBoundaryTest.kt
@@ -5,6 +5,8 @@ import org.junit.jupiter.api.Assertions.assertEquals
 import org.junit.jupiter.api.Assertions.assertTrue
 import org.junit.jupiter.api.Test
 import java.lang.reflect.Modifier
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
 
 /**
  * Nothing that goes to the server or to another radio amateur has got into the texts.
@@ -17,7 +19,7 @@ import java.lang.reflect.Modifier
  * deliberately not: a partial file is the normal state of a contribution in progress, and a
  * check over every language would turn the arrival of one into a red build.
  */
-class TranslationBoundaryTest {
+class TranslationBoundaryTest(@TempDir private val tempDir: Path) {
 
     private val everyText: List<String> =
         Translations.BY_LANGUAGE.values.flatMap { it.values }
@@ -63,7 +65,7 @@ class TranslationBoundaryTest {
 
     @Test
     fun noTextIsOneOfThePreferenceDefaultsThatGoToOtherRadioAmateurs() {
-        val prefs = ChatPreferences()
+        val prefs = ChatPreferences(tempDir.resolve("preferences.xml"))
 
         val defaults = ChatPreferences::class.java.declaredFields
             .filter { it.type == String::class.java && !Modifier.isStatic(it.modifiers) }
diff --git a/build.gradle.kts b/build.gradle.kts
index 9832afd0..636abeb8 100644
--- a/build.gradle.kts
+++ b/build.gradle.kts
@@ -68,8 +68,80 @@ subprojects {
         }
     }
 
+    /*
+     * Tests never see the operator's real home directory. ApplicationFileUtils resolves the
+     * application folder below user.home, and the no-argument ChatPreferences constructor
+     * seeds preferences.xml there when it is missing; a test that slipped through would
+     * create or rewrite files in ~/.praktiKST of whoever runs the build. Each test task's JVM
+     * gets its own scratch home, build/test-user-home/<task name>, emptied before the task
+     * runs, so one test task can never wipe another's. Tests that need their own profile
+     * directory still set user.home themselves and restore it afterwards.
+     *
+     * The redirection alone would hide the mistake, so every test task is finalized by
+     * checkTestUserHome, which fails the build if a preferences.xml was left in any scratch
+     * home: tests are expected to use ChatPreferences(Path) with a @TempDir. It is a
+     * finalizer and not a doLast because a doLast is skipped when a test fails, and a
+     * failing test must not hide the stray file. The check stays deliberately narrow: other
+     * files land in the scratch .praktiKST by design, for example the sound files
+     * PlayAudioUtils seeds from the bundled resources, and absorbing those is what the
+     * redirection is for. Only preferences.xml marks a test that should have used its own file.
+     *
+     * The opt-in ON4KST practice tests, which deliberately read the real profile, need
+     * -Don4kst.live=true, which the test task does not forward; they skip here either way
+     * and are meant to be run from an IDE.
+     */
+    val testUserHomeRoot = layout.buildDirectory.dir("test-user-home").get().asFile
+    val checkTestUserHome = tasks.register("checkTestUserHome") {
+        description = "Fails if a test wrote preferences.xml into its scratch user.home"
+        group = "verification"
+        doLast {
+            val strayPreferences = testUserHomeRoot.listFiles().orEmpty()
+                .map { it.resolve(".praktiKST/preferences.xml") }
+                .filter { it.exists() }
+            if (strayPreferences.isNotEmpty()) {
+                throw GradleException(
+                    "A test wrote ${strayPreferences.joinToString()}: it used the user " +
+                        "profile instead of ChatPreferences(Path) with a @TempDir"
+                )
+            }
+        }
+    }
+
     tasks.withType<Test>().configureEach {
         useJUnitPlatform()
+
+        val testUserHome = testUserHomeRoot.resolve(name)
+        /*
+         * The scratch home is a declared output. Test tasks are cacheable, and a cache hit
+         * skips doFirst; as an output the directory is restored together with the cached
+         * result, so checkTestUserHome always judges the files that belong to these inputs
+         * and never a leftover from another run.
+         *
+         * Skiko unpacks its native library (about 30 MB) below user.home unless told
+         * otherwise. It is kept beside the scratch home instead, so it neither bloats every
+         * cache entry nor is unpacked again on each run.
+         *
+         * Both paths are passed through an argument provider rather than systemProperty():
+         * a system property is a task input, and an absolute path there would make the
+         * cache key differ between checkouts.
+         */
+        val skikoDataPath = layout.buildDirectory.dir("skiko-data").get().asFile
+        outputs.dir(testUserHome)
+        jvmArgumentProviders.add(CommandLineArgumentProvider {
+            listOf(
+                "-Duser.home=${testUserHome.absolutePath}",
+                "-Dskiko.data.path=${skikoDataPath.absolutePath}"
+            )
+        })
+        doFirst {
+            // deleteRecursively() reports failure instead of throwing. A file left over
+            // from an earlier run would later be blamed on this one, so stop here.
+            if (!testUserHome.deleteRecursively()) {
+                throw GradleException("Could not empty the test user.home $testUserHome")
+            }
+            testUserHome.mkdirs()
+        }
+        finalizedBy(checkTestUserHome)
     }
 
     // XML for CI to read, HTML for people. Gradle does not wire the report after the test
diff --git a/core/src/main/java/kst4contest/controller/DXClusterThreadPooledServer.java b/core/src/main/java/kst4contest/controller/DXClusterThreadPooledServer.java
index e776a1ed..9c283d8f 100644
--- a/core/src/main/java/kst4contest/controller/DXClusterThreadPooledServer.java
+++ b/core/src/main/java/kst4contest/controller/DXClusterThreadPooledServer.java
@@ -40,7 +40,8 @@ public class DXClusterThreadPooledServer implements Runnable {
             Executors.newSingleThreadScheduledExecutor();
 
     private volatile boolean stopped;
-    private ServerSocket serverSocket;
+    /** Assigned by run() on the server thread and closed by stop() from another one. */
+    private volatile ServerSocket serverSocket;
 
     public DXClusterThreadPooledServer(
             int port,
diff --git a/core/src/main/java/kst4contest/controller/WinTestSkedSender.java b/core/src/main/java/kst4contest/controller/WinTestSkedSender.java
index 0f8a08b1..db10545b 100644
--- a/core/src/main/java/kst4contest/controller/WinTestSkedSender.java
+++ b/core/src/main/java/kst4contest/controller/WinTestSkedSender.java
@@ -178,7 +178,6 @@ public class WinTestSkedSender {
     private void sendUdp(WinTestMessage msg) throws Exception {
         try (DatagramSocket socket = new DatagramSocket()) {
             socket.setBroadcast(true);
-            socket.setReuseAddress(true);
 
             byte[] bytes = msg.toBytes();
             DatagramPacket packet = new DatagramPacket(bytes, bytes.length, broadcastAddress, port);
diff --git a/core/src/test/java/kst4contest/controller/On4KstConnectionProbeTest.java b/core/src/test/java/kst4contest/controller/On4KstConnectionProbeTest.java
index 28946a69..eaf13612 100644
--- a/core/src/test/java/kst4contest/controller/On4KstConnectionProbeTest.java
+++ b/core/src/test/java/kst4contest/controller/On4KstConnectionProbeTest.java
@@ -22,15 +22,20 @@ import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicLong;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 import org.junit.jupiter.api.Timeout;
 
 import kst4contest.model.ChatCategory;
 import kst4contest.model.ChatMessage;
 import kst4contest.model.ChatPreferences;
 import kst4contest.testing.ManualScheduler;
+import java.nio.file.Path;
 
 class On4KstConnectionProbeTest {
 
+    @TempDir
+    Path tempDir;
+
     @Test
     void separatesClientProbeServerProbeAndInternalResponses() {
         assertEquals("CK|", On4KstProtocol.clientLivenessProbe());
@@ -357,7 +362,7 @@ class On4KstConnectionProbeTest {
     }
 
     private ChatPreferences localPreferences() {
-        ChatPreferences preferences = new ChatPreferences();
+        ChatPreferences preferences = new ChatPreferences(tempDir.resolve("preferences.xml"));
         preferences.setStn_on4kstServersDns("127.0.0.1");
         preferences.setStn_loginCallSign("DL1ABC");
         preferences.setStn_loginPassword("test-password");
diff --git a/core/src/test/java/kst4contest/controller/On4KstProtocolTest.java b/core/src/test/java/kst4contest/controller/On4KstProtocolTest.java
index 5b91a72b..98b17906 100644
--- a/core/src/test/java/kst4contest/controller/On4KstProtocolTest.java
+++ b/core/src/test/java/kst4contest/controller/On4KstProtocolTest.java
@@ -5,13 +5,19 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 import java.time.LocalDateTime;
 import java.time.ZoneOffset;
 import kst4contest.model.ChatPreferences;
+import java.nio.file.Path;
 
 
 class On4KstProtocolTest {
+
+    @TempDir
+    Path tempDir;
+
     @Test
     void buildsLoginWithReplayOverlap() {
         assertEquals(
@@ -72,7 +78,7 @@ class On4KstProtocolTest {
 
     @Test
     void resolvesBeaconVariablesBeforeApplyingProtocolValidation() {
-        ChatPreferences preferences = new ChatPreferences();
+        ChatPreferences preferences = new ChatPreferences(tempDir.resolve("preferences.xml"));
         preferences.setMYQRGFirstCat("144.300");
 
         ChatController controller = new ChatController();
@@ -92,7 +98,7 @@ class On4KstProtocolTest {
 
     @Test
     void acceptsTemporarilyUnresolvedVariableOnlyBeaconTemplate() {
-        ChatPreferences preferences = new ChatPreferences();
+        ChatPreferences preferences = new ChatPreferences(tempDir.resolve("preferences.xml"));
         preferences.setMYQRGFirstCat("");
 
         ChatController controller = new ChatController();
@@ -110,7 +116,7 @@ class On4KstProtocolTest {
 
     @Test
     void rejectsEmptyOverlongAndProtocolBreakingBeaconText() {
-        ChatPreferences preferences = new ChatPreferences();
+        ChatPreferences preferences = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         ChatController controller = new ChatController();
         controller.setChatPreferences(preferences);
diff --git a/core/src/test/java/kst4contest/model/ChatPreferencesLanguageTest.java b/core/src/test/java/kst4contest/model/ChatPreferencesLanguageTest.java
index 2343c6c8..e81c2975 100644
--- a/core/src/test/java/kst4contest/model/ChatPreferencesLanguageTest.java
+++ b/core/src/test/java/kst4contest/model/ChatPreferencesLanguageTest.java
@@ -1,21 +1,26 @@
 package kst4contest.model;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import java.nio.file.Path;
 
 /** The interface language is stored per profile, like every other GUI preference. */
 class ChatPreferencesLanguageTest {
 
+    @TempDir
+    Path tempDir;
+
     @Test
     void theDefaultIsTheSystemLanguage() {
         // Empty means "follow the system", which is what the spec asks for on a first start.
-        assertEquals("", new ChatPreferences().getGuiOptions_language());
+        assertEquals("", new ChatPreferences(tempDir.resolve("preferences.xml")).getGuiOptions_language());
     }
 
     @Test
     void aChosenLanguageIsKept() {
-        ChatPreferences prefs = new ChatPreferences();
+        ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         prefs.setGuiOptions_language("de");
 
@@ -24,7 +29,7 @@ class ChatPreferencesLanguageTest {
 
     @Test
     void aNullSetterValueBecomesEmpty() {
-        ChatPreferences prefs = new ChatPreferences();
+        ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         prefs.setGuiOptions_language(null);
 
diff --git a/core/src/test/java/kst4contest/model/ChatPreferencesLocaleTest.java b/core/src/test/java/kst4contest/model/ChatPreferencesLocaleTest.java
index 7ae47efb..7e301ded 100644
--- a/core/src/test/java/kst4contest/model/ChatPreferencesLocaleTest.java
+++ b/core/src/test/java/kst4contest/model/ChatPreferencesLocaleTest.java
@@ -1,6 +1,7 @@
 package kst4contest.model;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 import java.lang.reflect.Field;
 import java.lang.reflect.Modifier;
@@ -10,6 +11,7 @@ import java.util.Map;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import java.nio.file.Path;
 
 /**
  * No default that goes to another radio amateur changes with the interface language.
@@ -22,11 +24,14 @@ import static org.junit.jupiter.api.Assertions.assertTrue;
  */
 class ChatPreferencesLocaleTest {
 
+    @TempDir
+    Path tempDir;
+
     private Map<String, String> stringFieldsUnder(final Locale locale) throws Exception {
         Locale previous = Locale.getDefault();
         try {
             Locale.setDefault(locale);
-            ChatPreferences prefs = new ChatPreferences();
+            ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
             Map<String, String> values = new LinkedHashMap<>();
 
             for (Field field : ChatPreferences.class.getDeclaredFields()) {
diff --git a/core/src/test/java/kst4contest/model/ChatPreferencesObservableTest.java b/core/src/test/java/kst4contest/model/ChatPreferencesObservableTest.java
index b7ad4b9b..0cba89a7 100644
--- a/core/src/test/java/kst4contest/model/ChatPreferencesObservableTest.java
+++ b/core/src/test/java/kst4contest/model/ChatPreferencesObservableTest.java
@@ -1,16 +1,21 @@
 package kst4contest.model;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import java.nio.file.Path;
 
 class ChatPreferencesObservableTest {
 
+    @TempDir
+    Path tempDir;
+
     @Test
     void theDefaultsMatchTheFormerJavaFxProperties() {
-        ChatPreferences prefs = new ChatPreferences();
+        ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         assertEquals(360.0, prefs.getActualQTF().get(), 0.0001,
                 "actualQTF defaulted to 360 as a SimpleDoubleProperty");
@@ -20,7 +25,7 @@ class ChatPreferencesObservableTest {
 
     @Test
     void theQrgValuesStartUnavailable() {
-        ChatPreferences prefs = new ChatPreferences();
+        ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         assertNull(prefs.getMYQRGFirstCat().get(),
                 "an unset own QRG must stay unavailable rather than becoming a fixed band");
@@ -29,7 +34,7 @@ class ChatPreferencesObservableTest {
 
     @Test
     void theListSettingsStartEmpty() {
-        ChatPreferences prefs = new ChatPreferences();
+        ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         assertTrue(prefs.getLstNotify_QSOSniffer_sniffedCallSignList().snapshot().isEmpty());
         assertTrue(prefs.getLstNotify_QSOSniffer_sniffedPrefixLocList().snapshot().isEmpty());
diff --git a/core/src/test/java/kst4contest/model/ChatPreferencesPaletteTest.java b/core/src/test/java/kst4contest/model/ChatPreferencesPaletteTest.java
index 24739277..2ace6345 100644
--- a/core/src/test/java/kst4contest/model/ChatPreferencesPaletteTest.java
+++ b/core/src/test/java/kst4contest/model/ChatPreferencesPaletteTest.java
@@ -1,8 +1,10 @@
 package kst4contest.model;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import java.nio.file.Path;
 
 /**
  * The two designs' colour overrides are stored independently.
@@ -13,9 +15,12 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
  */
 class ChatPreferencesPaletteTest {
 
+    @TempDir
+    Path tempDir;
+
     @Test
     void theTwoDesignsAreStoredIndependently() {
-        ChatPreferences prefs = new ChatPreferences();
+        ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         prefs.setGuiOptions_paletteOverridesDay("surface=#FFFFFF");
         prefs.setGuiOptions_paletteOverridesEvening("surface=#000000");
@@ -27,7 +32,7 @@ class ChatPreferencesPaletteTest {
     @Test
     void anUnsetOverrideReadsAsEmptyAndNotAsNull() {
         // A preferences file from an older release has no such element at all.
-        ChatPreferences prefs = new ChatPreferences();
+        ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         assertEquals("", prefs.getGuiOptions_paletteOverridesDay());
         assertEquals("", prefs.getGuiOptions_paletteOverridesEvening());
@@ -35,7 +40,7 @@ class ChatPreferencesPaletteTest {
 
     @Test
     void aNullSetterValueBecomesEmpty() {
-        ChatPreferences prefs = new ChatPreferences();
+        ChatPreferences prefs = new ChatPreferences(tempDir.resolve("preferences.xml"));
 
         prefs.setGuiOptions_paletteOverridesDay(null);
         prefs.setGuiOptions_paletteOverridesEvening(null);
diff --git a/core/src/test/java/kst4contest/test/ChatPreferencesStationMapVisibilityTest.java b/core/src/test/java/kst4contest/test/ChatPreferencesStationMapVisibilityTest.java
index 23f82912..d433867b 100644
--- a/core/src/test/java/kst4contest/test/ChatPreferencesStationMapVisibilityTest.java
+++ b/core/src/test/java/kst4contest/test/ChatPreferencesStationMapVisibilityTest.java
@@ -18,7 +18,8 @@ class ChatPreferencesStationMapVisibilityTest {
 
     @Test
     void pathAnalysisIsVisibleByDefault() {
-        ChatPreferences preferences = new ChatPreferences();
+        ChatPreferences preferences = new ChatPreferences(
+                temporaryDirectory.resolve("preferences.xml"));
 
         assertTrue(preferences.isGUIstationMapPathAnalysisVisible());
     }
@@ -27,9 +28,7 @@ class ChatPreferencesStationMapVisibilityTest {
     void hiddenPathAnalysisStateSurvivesXmlRoundTrip() throws IOException {
         Path preferencesFile = temporaryDirectory.resolve("preferences.xml");
 
-        ChatPreferences writtenPreferences = new ChatPreferences();
-        writtenPreferences.setStoreAndRestorePreferencesFileName(
-                preferencesFile.toString());
+        ChatPreferences writtenPreferences = new ChatPreferences(preferencesFile);
         writtenPreferences.setGUIstationMapPathAnalysisVisible(false);
         writtenPreferences.writePreferencesToXmlFile();
 
@@ -38,9 +37,7 @@ class ChatPreferencesStationMapVisibilityTest {
                 "<GUIstationMapPathAnalysisVisible>false"
                         + "</GUIstationMapPathAnalysisVisible>"));
 
-        ChatPreferences restoredPreferences = new ChatPreferences();
-        restoredPreferences.setStoreAndRestorePreferencesFileName(
-                preferencesFile.toString());
+        ChatPreferences restoredPreferences = new ChatPreferences(preferencesFile);
         restoredPreferences.readPreferencesFromXmlFile();
 
         assertFalse(restoredPreferences.isGUIstationMapPathAnalysisVisible());
@@ -63,9 +60,8 @@ class ChatPreferencesStationMapVisibilityTest {
                 </praktiKST>
                 """);
 
-        ChatPreferences restoredPreferences = new ChatPreferences();
-        restoredPreferences.setStoreAndRestorePreferencesFileName(
-                legacyPreferencesFile.toString());
+        ChatPreferences restoredPreferences =
+                new ChatPreferences(legacyPreferencesFile);
         restoredPreferences.readPreferencesFromXmlFile();
 
         assertTrue(restoredPreferences.isGUIstationMapPathAnalysisVisible());