android, desktop: refine RTL fix and add design doc

Surgical refinements on top of the initial RTL fix, plus a design doc
in plans/.

- chatItemShape (ChatItemView.kt): extract `val tailOnRight = ...` so
  the predicate names what the conditional does. Comment trimmed to
  the single load-bearing invariant ("default path draws tail at
  bottom-left") since the rest is implied by the variable name.

- SubscriptionStatusIcon.kt: hoist `val iconModifier =
  modifier.mirrorIfRtl()` once at the top of the composable and reuse
  it at all five Icon call sites. Calling mirrorIfRtl five times or
  once produces the same Modifier within one composition.

- GoToItemInnerButton (ChatItemView.kt): add `mirror: Boolean = false`
  parameter and pass `mirror = true` at the directional call site
  (ic_arrow_forward, "go to forwarded source"). The other call site
  (ic_search) keeps the default and is unchanged.

- Modifiers.kt, FramedItemView.kt: drop two explanatory comments that
  restated framework-primitive behavior (mirrorIfRtl's own name, and
  the documented behavior of placeRelative).

plans/2026-05-13-rtl-layout-issues.md documents the three independent
mechanisms behind the fix (bubble shape, app-bar slot placement,
asymmetric drawables), the bubble-tail XOR predicate, and four
considered-and-rejected alternatives.

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/views/chat/item/ChatItemView.kt

@@ -238,7 +238,7 @@ fun ChatItemView(
     }
 
     @Composable
-    fun GoToItemInnerButton(alignStart: Boolean, icon: ImageResource, iconSize: Dp = 22.dp, parentActivated: State<Boolean>, onClick: () -> Unit) {
+    fun GoToItemInnerButton(alignStart: Boolean, icon: ImageResource, iconSize: Dp = 22.dp, parentActivated: State<Boolean>, mirror: Boolean = false, onClick: () -> Unit) {
       val buttonInteractionSource = remember { MutableInteractionSource() }
       val buttonHovered = buttonInteractionSource.collectIsHoveredAsState()
       val buttonPressed = buttonInteractionSource.collectIsPressedAsState()
@@ -273,7 +273,7 @@ fun ChatItemView(
           .size(22.dp),
         interactionSource = buttonInteractionSource
       ) {
-        Icon(painterResource(icon), null, Modifier.size(iconSize), tint = iconTint)
+        Icon(painterResource(icon), null, Modifier.size(iconSize).then(if (mirror) Modifier.mirrorIfRtl() else Modifier), tint = iconTint)
       }
     }
 
@@ -289,7 +289,7 @@ fun ChatItemView(
           }
         }
       } else if (chatTypeApiIdMsgId != null) {
-        GoToItemInnerButton(alignStart, MR.images.ic_arrow_forward, 22.dp, parentActivated) {
+        GoToItemInnerButton(alignStart, MR.images.ic_arrow_forward, 22.dp, parentActivated, mirror = true) {
           val (chatType, apiId, msgId) = chatTypeApiIdMsgId
           withBGApi {
             openChat(secondaryChatsCtx = null, rhId, chatType, apiId, msgId)
@@ -1283,10 +1283,9 @@ private fun chatItemShape(roundness: Float, density: Density, tailVisible: Boole
     quadraticBezierTo(bubbleInitialX, 0f, bubbleInitialX + rx, 0f) // Top-left corner
   }
 
-  // The default path draws the tail at the bottom-left corner. We mirror the path so the tail
-  // ends up on the side closer to the message author's profile in chat: right for sent in LTR,
-  // left for received in LTR — and the opposite under RTL where chat sides are mirrored.
-  if (sent != (layoutDirection == LayoutDirection.Rtl)) {
+  // Default path draws the tail at the bottom-left corner.
+  val tailOnRight = sent != (layoutDirection == LayoutDirection.Rtl)
+  if (tailOnRight) {
     val matrix = Matrix()
     matrix.scale(-1f, 1f)
     this.transform(matrix)

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/views/chat/item/FramedItemView.kt

@@ -592,8 +592,6 @@ fun CenteredRowLayout(
     val second = measureable[1].measure(constraints.copy(minWidth = 0, minHeight = 0, maxWidth = (constraints.maxWidth - first.measuredWidth - third.measuredWidth).coerceAtLeast(0)))
     // Limit width for every other element to width of important element and height for a sum of all elements.
     layout(constraints.maxWidth, constraints.maxHeight) {
-      // placeRelative mirrors x under RTL so the leading slot (first) and trailing slot (third)
-      // swap visual sides automatically when the locale flips.
       first.placeRelative(0, ((constraints.maxHeight - first.measuredHeight) / 2).coerceAtLeast(0))
       second.placeRelative((constraints.maxWidth - second.measuredWidth) / 2, ((constraints.maxHeight - second.measuredHeight) / 2).coerceAtLeast(0))
       third.placeRelative(constraints.maxWidth - third.measuredWidth, ((constraints.maxHeight - third.measuredHeight) / 2).coerceAtLeast(0))

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/views/helpers/Modifiers.kt

@@ -12,8 +12,6 @@ import androidx.compose.ui.unit.IntOffset
 import androidx.compose.ui.unit.LayoutDirection
 import kotlin.math.roundToInt
 
-// Horizontally flips an asymmetric directional drawable (back/forward arrows, chevrons, signal
-// waves) when the layout direction is RTL. Apply to the Icon's modifier at each call site.
 @Composable
 fun Modifier.mirrorIfRtl(): Modifier =
   if (LocalLayoutDirection.current == LayoutDirection.Rtl) this.scale(scaleX = -1f, scaleY = 1f) else this

apps/multiplatform/common/src/commonMain/kotlin/chat/simplex/common/views/helpers/SubscriptionStatusIcon.kt

@@ -14,28 +14,29 @@ fun SubscriptionStatusIcon(
   variableValue: Float,
   modifier: Modifier = Modifier
 ) {
+  val iconModifier = modifier.mirrorIfRtl()
   @Composable
   fun ZeroIcon() {
-    Icon(painterResource(MR.images.ic_radiowaves_up_forward_4_bar), null, tint = color.copy(alpha = 0.33f), modifier = modifier.mirrorIfRtl())
+    Icon(painterResource(MR.images.ic_radiowaves_up_forward_4_bar), null, tint = color.copy(alpha = 0.33f), modifier = iconModifier)
   }
 
   when {
     variableValue <= 0f -> ZeroIcon()
     variableValue > 0f && variableValue <= 0.25f -> Box {
       ZeroIcon()
-      Icon(painterResource(MR.images.ic_radiowaves_up_forward_1_bar), null, tint = color, modifier = modifier.mirrorIfRtl())
+      Icon(painterResource(MR.images.ic_radiowaves_up_forward_1_bar), null, tint = color, modifier = iconModifier)
     }
 
     variableValue > 0.25f && variableValue <= 0.5f -> Box {
       ZeroIcon()
-      Icon(painterResource(MR.images.ic_radiowaves_up_forward_2_bar), null, tint = color, modifier = modifier.mirrorIfRtl())
+      Icon(painterResource(MR.images.ic_radiowaves_up_forward_2_bar), null, tint = color, modifier = iconModifier)
     }
 
     variableValue > 0.5f && variableValue <= 0.75f -> Box {
       ZeroIcon()
-      Icon(painterResource(MR.images.ic_radiowaves_up_forward_3_bar), null, tint = color, modifier = modifier.mirrorIfRtl())
+      Icon(painterResource(MR.images.ic_radiowaves_up_forward_3_bar), null, tint = color, modifier = iconModifier)
     }
 
-    else -> Icon(painterResource(MR.images.ic_radiowaves_up_forward_4_bar), null, tint = color, modifier = modifier.mirrorIfRtl())
+    else -> Icon(painterResource(MR.images.ic_radiowaves_up_forward_4_bar), null, tint = color, modifier = iconModifier)
   }
 }