Windows: spooky ghost of closed "About" window appears sometimes

Expected Behavior

Closed windows should not exist.

Actual Behavior

If the About window has been opened and then closed, it can appear on the screen when switching between apps.

Steps to Reproduce

1. Make sure at least one application other than Scratch is running and is shown on the Windows task bar.
2. Start the Scratch app
3. Click the Scratch logo to open the "About" window
4. Click the "X" to close the "About" window
5. Switch to some other application
6. Do any of:
   • Hold the Alt key and tap tab until the Scratch app is selected. Continue holding Alt for a few seconds: the ghost of the "About" window will appear on the screen.
   • Hover the mouse over the Scratch app icon in the task bar; a small image of the Scratch app window should pop up. Once it does, hover the mouse over that small image (see screenshot below). Other app windows will disappear and the ghost of the "About" window will appear.

Operating System and Browser

Windows 10

No source file or test is named in the issue. First reproduce the closed About-window ghost on Windows 10 using the listed task-switching and task-bar steps, then trace the desktop window lifecycle; done means the closed About window no longer appears during either interaction.