Regression in [0] which exposed a problem with GHOST_kGrabHide on Win32
and to some extent X11.
Prior to [0], walk mode used it's own warping logic (hiding the cursor
& recording the motion between events). Using GHOST's grabbing makes
sense in this case as it's not very convenient for operators to
implement their own cursor warping, however doing so exposed a problem
where the mouse cursor could leave the window.
This would happen because the cursor needed to be within 2px of the
screen edge before warping.
Resolve by warping within a small region in the middle of the window.
Note that warping to the window center on each motion would be ideal
but is more involved as the logic for Win32 & X11 doesn't work properly
when every motion warps, so this needs further investigation to support.
This problem doesn't apply to GHOST/Cocoa which warps every motion event
on the spot and GHOST/Wayland doesn't set the mouse position at all to
implement this functionality.
[0]: 4c4e8cc926
9fd6dae793