Fix the tab rail flickering on narrow iPad windows

The rail is laid out beside the split view, so hiding it when the split
collapsed handed that width back to the split, which then had room for its
sidebar again, which showed the rail, which collapsed the split. Any window
whose width sat within a rail of the split's collapse threshold flickered
forever, an iPad mini in fullscreen among them.

The split reports the same column visibility whether it collapsed on its own or
because the sidebar was toggled, and at the same width, so the two can only be
told apart by when they arrive: the rebound lands in the next layout pass. A
collapse that soon after showing the rail is therefore taken as proof that the
rail and the sidebar don't fit at this width, and the rail is left hidden until
the window is resized. Later collapses are treated as deliberate and still hide
the rail as before.

Author: stefanceriu

ElementX/Sources/Application/Navigation/NavigationTabCoordinator.swift

@@ -317,6 +317,11 @@ 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 width at which the rail and the split view's sidebar have been shown not to fit together.
+    @State private var railBlockedWidth: CGFloat?
+    /// When the rail was last shown, used to spot the split view collapsing as a direct result of it.
+    @State private var railShownDate = Date.distantPast
 
     var body: some View {
         tabView
@@ -385,8 +390,13 @@ 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
+            if size.width != containerWidth {
+                containerWidth = size.width
+                railBlockedWidth = nil // The rail and the sidebar may well fit at the new width.
+            }
+            
             guard let newValue = window?.isFullScreen else { return }
 
             if newValue != isFullScreen {
@@ -399,7 +409,21 @@ 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 }
+            DispatchQueue.main.async {
+                if selectedTabSplitColumnVisibility == .detailOnly {
+                    // A collapse this soon after showing the rail was caused by the rail's own width, so
+                    // showing it again would only collapse the split view again, flickering forever. Note
+                    // that the two don't fit at this width and leave the rail hidden until that changes.
+                    if railShownDate.timeIntervalSinceNow > -0.5 {
+                        railBlockedWidth = containerWidth
+                    }
+                    
+                    isRailVisible = false
+                } else if !isRailVisible, railBlockedWidth != containerWidth {
+                    isRailVisible = true
+                    railShownDate = .now
+                }
+            }
         }
     }