The problem is the one described here:
Essentially when there are many (for example two) canvases open, and one closes, via the red 'X' button, some but not all of them (only one, in the two total canvases example), when later one quits root (.q or Browser->Quit Root), there's a crash.
Closing the canvases using TCanvas::Close() seems not triggering the problem.
The minimal steps to reproduce the issue (confirmed by several people, as shown in the ROOT forum post) is to:
- open 2 canvases
- close the second via the red 'X' button
- close ROOT