This is a longstanding minor issue.
I have the Anki icon in the Windows toolbar at the bottom of the screen.
Doing a mouseover of the icon (only while the Anki program is running) causes live thumbnails of all open Anki windows to appear just above the toolbar, in particular the main Anki window (Decks, Add, Browse, Stats, Sync).
Then doing a mouseover of the thumbnail of the main Anki window causes every single old sync popup window that has ever run since Anki was launched to reappear in their original screen positions (even if the main Anki window itself has been moved). The sync windows show “Checking…” “Added/modified” “Removed”, and then there is also a “Processing…” window from an add-on that does QueryOp using with_progress().
All those old popup windows disappear when the cursor is moved and the mouseover ends, and reappear whenever the mouse cursor moves back over the thumbnail.
In the screenshot image below, imagine the mouse cursor being on top of the thumbnail. The fact that the mouse cursor is not visible is an artifact of the screenshot capture process.
The persistence of these popup windows suggests that resources aren’t being freed, which would be a memory leak, albeit probably minor.