49e4923384
When clicking or dragging a Panel's drag handle, a PANEL_STATE_DRAG
state transition is followed immediately by a PANEL_STATE_ANIMATE
transition. Both transitions start a timer.
Unfortunately, a subtle change introduced in
f3ab698951 meant the first timer would no
longer be destroyed before starting the second one.
The fix is to just reuse the timer if it's already present so that a
second one is not created in the first place.
Pull Request: https://projects.blender.org/blender/blender/pulls/113711