fix: avoid memory usage growth over time (closes #5138)

src/logic/Windows.swift

@@ -521,6 +521,11 @@ class Windows {
     }
 
     static func removeWindows(_ windows: [Window], _ addWindowlessWindowIfNeeded: Bool) {
+        for w in windows {
+            if w.application.focusedWindow?.cgWindowId == w.cgWindowId {
+                w.application.focusedWindow = nil
+            }
+        }
         let toRemove = windows.map { $0.lastFocusOrder }
         list.removeAll { w in
             if toRemove.contains(w.lastFocusOrder) {

src/ui/main-window/TilesView.swift

@@ -436,9 +436,10 @@ class TilesView {
                 rowSignature.append(index)
                 window.rowIndex = rows.count - 1
             } else {
-                // release images from unused recycledViews; they take lots of RAM
+                // release images and stale window references from unused recycledViews; they take lots of RAM
                 view.thumbnail.releaseImage()
                 view.appIcon.releaseImage()
+                view.window_ = nil
             }
         }
         scrollView.documentView!.subviews = newViews