Unfortunately, I can not always rely on transient hint, so use another fix found today (uncomment the code).

Sometimes TRootContextMenu is created without canvas (as Bertrand demonstrated with a TBrowser and context menu for
a selected tree view item). So the only thing I can do now - try to preserve z-order for my windows.
Ah??
Merge branch 'master' of https://root.cern.ch/git/root