When changing the tooltip text or markup of widget A, we simulate a
motion event on that widget to update the display.
But if there is an active tooltip on widget B, then the current code
belives we moved the pointer from B to A thus hides/shows the tooltip
of B alternatively.

Instead, simulate the motion event on the widget currently under the pointer.
This will avoid flickering if we keep the pointer over widget B.

Fixes #6674
Closes #6674