A key held from the menu (Hold Ctrl, Alt or Windows) is the same key as
the keyboard's left Ctrl, Alt or Windows key. Pressing and releasing
that key on the screen let the VM's key go, while the menu still showed
it held. And when the key went down outside the screen (in the
Clipboard dialog) and came up on it, noVNC sent no release, as it had
not seen the key go down.
The release of that key where noVNC takes keys (the screen, the touch
keyboard) now lets a held key go itself, and clears the menu's mark. A
release elsewhere reaches no VM and changes nothing.
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