Fix the tab rail flickering on narrow iPad windows

The rail was hidden when the split view collapsed, but that freed up enough
width for the split to expand again, showing the rail and collapsing the split.
The split's state now only hides the rail and showing it again is driven by the
container's width growing.

Author: stefanceriu

ElementX/Sources/Application/Navigation/NavigationTabCoordinator.swift

@@ -317,6 +317,9 @@ private struct NavigationTabCoordinatorView<Tag: Hashable>: View {
     @State private var isFullScreen = true
     @State private var isRailVisible = true
     @State private var railBackgroundColor: Color = .compound.bgCanvasDefault
+    @State private var containerWidth: CGFloat = 0
+    /// The container's width at the moment the rail was hidden, used to avoid flickering. See ``tabRailLayout``.
+    @State private var railHiddenWidth: CGFloat?
 
     var body: some View {
         tabView
@@ -385,8 +388,16 @@ private struct NavigationTabCoordinatorView<Tag: Hashable>: View {
             }
         }
         .onGeometryChange(for: CGSize.self) { geometry in
-            geometry.size // We don't need the size, but it's a signal to re-compute.
-        } action: { _ in
+            geometry.size
+        } action: { size in
+            containerWidth = size.width
+            
+            // The container grew, so give the rail another chance at being shown.
+            if let railHiddenWidth, size.width > railHiddenWidth {
+                self.railHiddenWidth = nil
+                isRailVisible = true
+            }
+            
             guard let newValue = window?.isFullScreen else { return }
 
             if newValue != isFullScreen {
@@ -399,7 +410,17 @@ private struct NavigationTabCoordinatorView<Tag: Hashable>: View {
             // feedback loop that locks up the app while resizing.
             //
             // Use a stored value (instead of a computed value) that is updated on the next run loop to break the loop.
-            DispatchQueue.main.async { isRailVisible = selectedTabSplitColumnVisibility != .detailOnly }
+            //
+            // Only ever hide the rail from here too. Showing it again when the split reports .all would
+            // evaluate the split at its wider, rail-less width, which flip-flops forever whenever the
+            // container's width sits between the split's collapse threshold and that threshold plus the
+            // rail's width. Instead the rail is shown again above, once the container has actually grown.
+            guard selectedTabSplitColumnVisibility == .detailOnly else { return }
+            
+            DispatchQueue.main.async {
+                railHiddenWidth = containerWidth
+                isRailVisible = false
+            }
         }
     }