One of my students moved a sprite while in Stage mode. She was on the Background tab.
When the sprite moved it left behind an image of itself. The 'ghost' image was not a new sprite. Nor was the 'ghost' sprite image part of any of the backgrounds. When she double-clicked on the 'ghost' sprite image the Stage Background tab appeared suggesting that the mouse was responding to the underlying background image.
The student has recovered her program by exporting all of the sprites and backgrounds and importing them into a new project.
However, it would be interesting to know if there is an explanation for this event.
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