By default macOS allows you to open apps from the official Mac App Store only. If you have this still set as your default you will be seeing the warning when you try to open an app for the first time.

When App Exposé is open, simply tap on an instance of an app to open it (including the app alongside it if it's in Split Screen mode). Tag: iPadOS. Related Forum: iOS 13.

If Title Bar or controls of an open application window moves off-screen or window slides off the Windows 11/10 desktop, use this trick to move or close the application.