Open menu row menus flush under the bar (#143)

Material3's DropdownMenu clamps every popup to 48dp below the window's
top edge (MenuVerticalMargin in its position provider); the margin is
applied after the offset parameter and the provider is not replaceable,
so the in-window menu bar's menus all opened with a visible gap hanging
below the bar. Replace them with a plain focusable Popup in MenuDefaults
styling and an own position provider that puts the menu directly under
its anchor, flipping above only when there is no room below. The anchor
box takes the full bar height so the menu starts at the bar's bottom
edge, not at the centred title text.


Claude-Session: https://claude.ai/code/session_01BegdsGSNNxKR6VPNPNvfoc

Co-authored-by: Claude <noreply@anthropic.com>

app-desktop/src/main/kotlin/kst4contest/view/compose/MainMenuBar.kt

@@ -6,14 +6,20 @@ import androidx.compose.foundation.hoverable
 import androidx.compose.foundation.interaction.MutableInteractionSource
 import androidx.compose.foundation.interaction.collectIsHoveredAsState
 import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.IntrinsicSize
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.heightIn
 import androidx.compose.foundation.layout.padding
-import androidx.compose.material3.DropdownMenu
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.rememberScrollState
+import androidx.compose.foundation.verticalScroll
 import androidx.compose.material3.DropdownMenuItem
 import androidx.compose.material3.HorizontalDivider
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.MenuDefaults
+import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.getValue
@@ -21,8 +27,16 @@ import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.unit.IntOffset
+import androidx.compose.ui.unit.IntRect
+import androidx.compose.ui.unit.IntSize
+import androidx.compose.ui.unit.LayoutDirection
+import androidx.compose.ui.unit.dp
 import androidx.compose.ui.window.FrameWindowScope
 import androidx.compose.ui.window.MenuBar
+import androidx.compose.ui.window.Popup
+import androidx.compose.ui.window.PopupPositionProvider
+import androidx.compose.ui.window.PopupProperties
 import kst4contest.controller.On4KstConnectionState
 import kst4contest.utils.PlatformUtils
 import kst4contest.view.i18n.LocalStrings
@@ -163,7 +177,14 @@ private fun MenuTitle(menu: MenuSpec, rowState: MainMenuRowState) {
     val hovered by interactions.collectIsHoveredAsState()
     val open = rowState.openMenu == menu.title
 
-    Box {
+    /*
+     * Full bar height, so the popup anchored to this box starts at the bar's bottom edge
+     * rather than at the centred title text a few pixels above it.
+     */
+    Box(
+        modifier = Modifier.heightIn(min = Density.TAB_MIN_HEIGHT),
+        contentAlignment = Alignment.Center,
+    ) {
         Text(
             text = menu.title,
             style = MaterialTheme.typography.bodyMedium,
@@ -195,29 +216,90 @@ private fun MenuTitle(menu: MenuSpec, rowState: MainMenuRowState) {
                 ),
         )
 
-        DropdownMenu(expanded = open, onDismissRequest = rowState::close) {
-            menu.entries.forEach { entry ->
-                when (entry) {
-                    is MenuEntry.Item -> DropdownMenuItem(
-                        text = {
-                            Text(entry.label, style = MaterialTheme.typography.bodyMedium)
-                        },
-                        enabled = entry.enabled,
-                        onClick = { rowState.pick(entry.onClick) },
-                    )
+        /*
+         * A hand-rolled stand-in for Material3's DropdownMenu, because that one cannot open
+         * flush under a menu bar — see MenuRowPopupPositionProvider. Same dismissal (outside
+         * click and Escape come with a focusable popup), same MenuDefaults look; only the
+         * expand animation is gone, which a menu bar does not have anyway.
+         */
+        if (open) {
+            Popup(
+                popupPositionProvider = MenuRowPopupPositionProvider,
+                onDismissRequest = rowState::close,
+                properties = PopupProperties(focusable = true),
+            ) {
+                Surface(
+                    shape = MenuDefaults.shape,
+                    color = MenuDefaults.containerColor,
+                    tonalElevation = MenuDefaults.TonalElevation,
+                    shadowElevation = MenuDefaults.ShadowElevation,
+                ) {
+                    Column(
+                        modifier = Modifier
+                            .padding(vertical = MENU_POPUP_VERTICAL_PADDING)
+                            .width(IntrinsicSize.Max)
+                            .verticalScroll(rememberScrollState()),
+                    ) {
+                        menu.entries.forEach { entry ->
+                            when (entry) {
+                                is MenuEntry.Item -> DropdownMenuItem(
+                                    text = {
+                                        Text(entry.label, style = MaterialTheme.typography.bodyMedium)
+                                    },
+                                    enabled = entry.enabled,
+                                    onClick = { rowState.pick(entry.onClick) },
+                                )
 
-                    MenuEntry.Separator ->
-                        HorizontalDivider(
-                            thickness = Density.HAIRLINE,
-                            color = palette.separatorLine,
-                        )
+                                MenuEntry.Separator ->
+                                    HorizontalDivider(
+                                        thickness = Density.HAIRLINE,
+                                        color = palette.separatorLine,
+                                    )
+                            }
+                        }
+                    }
                 }
             }
         }
     }
 }
 
+/**
+ * Places a menu-bar menu: flush under its anchor, leading edges aligned.
+ *
+ * Material3's DropdownMenu cannot be told to do this. Its position provider refuses any
+ * position closer than 48 dp (`MenuVerticalMargin`) to the window's top edge, and a menu
+ * bar sits exactly there, so every menu opened pushed down to that margin — a visible gap
+ * below the bar. The margin is applied after the `offset` parameter and the provider is
+ * not replaceable, hence this one and the hand-rolled popup in [MenuTitle].
+ */
+internal object MenuRowPopupPositionProvider : PopupPositionProvider {
+    override fun calculatePosition(
+        anchorBounds: IntRect,
+        windowSize: IntSize,
+        layoutDirection: LayoutDirection,
+        popupContentSize: IntSize,
+    ): IntOffset {
+        val preferredX = when (layoutDirection) {
+            LayoutDirection.Ltr -> anchorBounds.left
+            LayoutDirection.Rtl -> anchorBounds.right - popupContentSize.width
+        }
+        val x = preferredX.coerceIn(0, maxOf(0, windowSize.width - popupContentSize.width))
+        val below = anchorBounds.bottom
+        val above = anchorBounds.top - popupContentSize.height
+        val y = when {
+            below + popupContentSize.height <= windowSize.height -> below
+            above >= 0 -> above
+            else -> maxOf(0, windowSize.height - popupContentSize.height)
+        }
+        return IntOffset(x, y)
+    }
+}
+
 /** Enough tint to show the pointer is over a title, not enough to read as a selection. */
 private const val HOVER_ALPHA = 0.35f
 
 private val MENU_TITLE_VERTICAL_PADDING = androidx.compose.ui.unit.Dp(4f)
+
+/** Material3's DropdownMenuVerticalPadding, which is internal; keep the menus looking alike. */
+private val MENU_POPUP_VERTICAL_PADDING = 8.dp