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

[cwillisf]

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

[cwillisf]

I think the best way to fix this will be to allow the "About" window's close event to run its default behavior instead of calling hide() and prevent the default behavior. The "hide" behavior was intended as an optimization (re-opening the window is faster if it's hidden instead of closed) but I think these side effects override that concern.

We'll need to make sure that pressing the "About" button twice doesn't open two copies of the "About" window, and that there aren't any problems opening and closing the "About" window repeatedly.