fix: enabling previews with multiple screens could lead to flickering

src/ui/main-window/PreviewPanel.swift

@@ -40,6 +40,12 @@ class PreviewPanel: NSPanel {
             }
             currentId = id
             order(.below, relativeTo: App.app.thumbnailsPanel.windowNumber)
+            // Despite using `previewPanel.order(.below)`, a z-ordering issue can occur in the following scenario:
+            // 1. Show a preview of a window that is on a different monitor than the thumbnails panel
+            // 2. Select a window in the switcher that is on the same monitor as the thumbnails panel, and whose position overlaps with the thumbnails panel
+            // 3. For a single frame, the preview of the newly selected window can appear above the thumbnails panel before going back underneath it
+            // Simply using order(.below) is not sufficient to prevent this brief flicker. We explicitly set the preview panel's window level to be one below the thumbnails panel
+            App.app.previewPanel.level = App.app.thumbnailsPanel.level - 1
         }
     }
 }