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If you do `C-h k ... mouse-1 in other frame` (at least if you have a focus
that follows the mouse), then additionally to the down-mouse-1 and mouse-1
events, a `switch-frame` event (and `select-window` event as well sometimes)
is generated.  When `read-key-sequence` is called with nil for
`can-return-switch-frame`, this event is not returned but kept for later,
which causes a subsequent `sit-for` to return nil immediately.
This interfered without our "wait for double-click" which in turn prevented
us from stopping after the mouse-1 click, getting stuck waiting for something
else instead.

(help--read-key-sequence): Pass a non-nil `can-return-switch-frame`, so the
subsequent `sit-for` returns more trustworthy information.