So far the FocusTracker of the OSK can only recognize grab ops on a
window, that is when the user grabs the window using a mouse or the
touchscreen and actively drags it somewhere.
Window can also be moved using keyboard shortcuts, fullscreen buttons or
other ways which don't rely on grabs. Start also supporting those window
movements by listening to the "position-changed" signal on the currently
focused window and emitting the new "window-moved" signal in that case.
Because the OSK sometimes moves windows by itself, we temporarily
disconnect from that new signal while we move the focused window in
_windowSlideAnimationComplete().
This also takes care of resetting this._focusWindowStartY on movements
of the window.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1760>
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