Frame markers on the ruler canvas are not actually accurate.

I have proved that in my "framestepping-pygi" branch (derived from an old patch in bug #573895):
1- load a project with my various clips
2- set the project settings' framerate to 23.976, zoom in fully
3- framestep between frames, then move further to the right in the timeline and you'll notice it's off sync from the blue keyframe markers when you click and press the framestepping keys again.

From checking with a simple calculator and a debug statement I put in the stepFrame method, the algorithm of stepFrame is correct... Also, when hovering over the timeline ruler, the tooltip confirms that stepFrame is correct and that the ruler representation is wrong.

The reason is, among other things, that the ruler uses relative paint positions instead of using absolute positions, and adds some small offset errors everytime.