Permalink polish (#2771)

* Fix unbalanced padding.

* Animate the scroll to an event when it is already loaded.

* Perform the timeline clearance immediately before replacing the items.
Hides the empty timeline that was briefly shown.

* Make switching timeline more resilient.

ElementX/Sources/Screens/RoomScreen/RoomScreenModels.swift

@@ -115,6 +115,8 @@ enum RoomScreenViewAction {
     case focusLive
     /// The timeline scrolled to reveal the focussed item.
     case scrolledToFocussedItem
+    /// The table view has loaded the first items for a new timeline.
+    case hasSwitchedTimeline
 }
 
 enum RoomScreenComposerAction {
@@ -227,10 +229,27 @@ struct TimelineViewState {
     var isLive = true
     var paginationState = PaginationState.default
     
-    /// The ID of the focussed event navigated to via a permalink.
-    var focussedEventID: String? { didSet { focussedEventNeedsDisplay = focussedEventID != nil } }
-    /// Whether the timeline should scroll to `focussedEventID` once its item has been built and added to the timeline.
-    var focussedEventNeedsDisplay: Bool
+    /// The room is in the process of loading items from a new timeline (switching to/from a detached timeline).
+    var isSwitchingTimelines = false
+    
+    struct FocussedEvent: Equatable {
+        enum Appearance {
+            /// The event should be shown using an animated scroll.
+            case animated
+            /// The event should be shown immediately, without any animation.
+            case immediate
+            /// The event has already been shown.
+            case hasAppeared
+        }
+
+        /// The ID of the event.
+        let eventID: String
+        /// How the event should be shown, or whether it has already appeared.
+        var appearance: Appearance
+    }
+    
+    /// A focussed event that was navigated to via a permalink.
+    var focussedEvent: FocussedEvent?
     
     // These can be removed when we have full swiftUI and moved as @State values in the view
     var scrollToBottomPublisher = PassthroughSubject<Void, Never>()