This longstanding bug is part of our secret plan to discourage people from trying to do drawing/painting with the mouse ;-)

It's got to do with initialization of low-pass filters - they initialize themselves whenever there is motion (and with a tablet, there is always motion).

1) start mypaint
2) Do NOT move mouse.
3) From current position very quickly draw line with mouse
4) Mypaint might add unexpected hook to the beginning of the line. (see screenshot: I've drawn vertical line, diagonal line was added by mypaint)

AFAICT this happens due mypaint-brush.c\ mypaint_brush_stroke_to due its usage of states at line 790:

fac is calculated from dtime and slow position tracking setting of brush(In my case it is 1.03 of deevad/pen).

Upon initialization states are zeroed, so calculated value is lower than it's supposed to be.

If mouse is moved before drawing, then motion_notify_cb at canvasevent.py:519 will call stroke_to with zero pressure, so coordinate states will be updated. However if stroke_to with pressure is called immediately without moving mouse, then weird thing might happen