The current gnome-shell (v42.2) has a bug where grabbing the cursor
doesn't scale the region when confining it to the window.
For Hi-DPI displays this means the cursor may be confined to a quarter
of the window, making grab unusable.
Even though this has been fixed up-stream the issue remains in the
latest release - so workaround the problem by implementing window
confined grab using a software cursor.
This is only used gnome-shell for displays that use Hi-DPI scaling.
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