diff --git a/src/apps/mobile/android/app/src/main/kotlin/com/openbitfun/mobile/app/ui/shell/OpenBitFunCompactDrawer.kt b/src/apps/mobile/android/app/src/main/kotlin/com/openbitfun/mobile/app/ui/shell/OpenBitFunCompactDrawer.kt
index c5a5f04ed4..2a7a8589e0 100644
--- a/src/apps/mobile/android/app/src/main/kotlin/com/openbitfun/mobile/app/ui/shell/OpenBitFunCompactDrawer.kt
+++ b/src/apps/mobile/android/app/src/main/kotlin/com/openbitfun/mobile/app/ui/shell/OpenBitFunCompactDrawer.kt
@@ -35,6 +35,7 @@ import com.openbitfun.mobile.app.ui.theme.MotionDrawerHideMillis
 import com.openbitfun.mobile.app.ui.theme.MotionDrawerOpenMillis
 import com.openbitfun.mobile.app.ui.theme.MotionDrawerRevealMillis
 import com.openbitfun.mobile.app.ui.theme.MotionDrawerScrimMillis
+import com.openbitfun.mobile.app.ui.theme.openBitFunColors
 import kotlinx.coroutines.coroutineScope
 import kotlinx.coroutines.flow.first
 import kotlinx.coroutines.launch
@@ -218,7 +219,21 @@ internal fun OpenBitFunCompactDrawer(
     }
 
     Box(Modifier.fillMaxSize()) {
+        // The open drawer's fill is a floor under the whole shell, not a panel
+        // the width of the sidebar. The content card's rounded corners have to
+        // curve onto something: a fill that stopped at the card's left edge
+        // left them curving onto the page white, so a square-cornered grey
+        // block sat against a rounded card with a white wedge between them.
+        // Driven by contentProgress rather than drawerProgress so the floor is
+        // never thinner than the card has already moved, in either direction.
         if (compact && drawerComposed) {
+            Box(
+                Modifier
+                    .fillMaxSize()
+                    .background(openBitFunColors.sidebar.background)
+                    .graphicsLayer { alpha = contentProgress.value },
+            )
+
             Box(
                 modifier = Modifier
                     .width(drawerWidth)
diff --git a/src/apps/mobile/harmonyos/entry/src/main/ets/pages/components/AppShell.ets b/src/apps/mobile/harmonyos/entry/src/main/ets/pages/components/AppShell.ets
index ef319b5dfd..3a22388991 100644
--- a/src/apps/mobile/harmonyos/entry/src/main/ets/pages/components/AppShell.ets
+++ b/src/apps/mobile/harmonyos/entry/src/main/ets/pages/components/AppShell.ets
@@ -12,6 +12,7 @@ import {
   PAGE_BG,
   SCRIM,
   SHELL_SCRIM,
+  SIDEBAR_BG,
   TRANSPARENT,
 } from './Theme';
 
@@ -48,6 +49,26 @@ export struct AppShell {
             }
           })
 
+      // The open drawer's fill is a floor under the whole shell, not a panel
+      // the width of the sidebar. The content card's rounded corners have to
+      // curve onto something: when the fill stopped at the card's left edge
+      // they curved onto the page white instead, so a square-cornered grey
+      // block sat against a rounded card with a white wedge between them.
+      Column()
+        .width('100%')
+        .height('100%')
+        .backgroundColor(SIDEBAR_BG)
+        .expandSafeArea([SafeAreaType.SYSTEM], [SafeAreaEdge.TOP, SafeAreaEdge.BOTTOM])
+        .hitTestBehavior(HitTestMode.None)
+        .opacity((this.allowSidebar && this.shellState.showSidebar) ? 1 : 0)
+        // Matched to the content card rather than to the sidebar panel: the
+        // card must never finish moving before the floor it curves onto has
+        // finished fading, in either direction.
+        .animation({
+          duration: (this.allowSidebar && this.shellState.showSidebar) ? 320 : 250,
+          curve: Curve.EaseOut
+        })
+
       Column() {
         // The drawer is a compact affordance. Wide layouts put the same session
         // list in the master pane instead, and this copy stayed mounted behind
diff --git a/src/apps/mobile/ios/OpenBitFun/Features/Shell/MobileShellView.swift b/src/apps/mobile/ios/OpenBitFun/Features/Shell/MobileShellView.swift
index 4bf74f7aea..00091ffaac 100644
--- a/src/apps/mobile/ios/OpenBitFun/Features/Shell/MobileShellView.swift
+++ b/src/apps/mobile/ios/OpenBitFun/Features/Shell/MobileShellView.swift
@@ -176,7 +176,25 @@ struct MobileShellView: View {
         let compactSidebarWidth = min(280, max(220, viewportWidth * 0.68))
 
         ZStack(alignment: .leading) {
+            // The open drawer's fill is a floor under the whole shell, not a
+            // panel the width of the sidebar. The content card's rounded
+            // corners have to curve onto something: a fill that stopped at the
+            // card's left edge left them curving onto the page white, so a
+            // square-cornered grey block sat against a rounded card with a
+            // white wedge between them.
             if !sidebarVisible {
+                OpenBitFunTheme.sidebarBg
+                    .ignoresSafeArea()
+                    .opacity(model.drawerOpen ? 1 : 0)
+                    .allowsHitTesting(false)
+                    // Matched to the content card rather than to the sidebar
+                    // panel: the card must never finish moving before the floor
+                    // it curves onto has finished fading, in either direction.
+                    .animation(
+                        .easeOut(duration: model.drawerOpen ? 0.32 : 0.25),
+                        value: model.drawerOpen
+                    )
+
                 SidebarView(model: model)
                     .frame(width: compactSidebarWidth)
                     .opacity(model.drawerOpen ? 1 : 0)