If OpenGL is disabled, some tools, like the ellipse tool leave traces of the tool decorations on the canvas. I guess there is a rounding issue in the update calls.
Git commit 9aabe12b581d54ae7b40f6959f23b6b52632024c by Boudewijn Rempt.
Committed on 18/12/2011 at 14:19.
Pushed by rempt into branch 'master'.
Ellipse tool: update a slightly larger part of the canvas
This avoids the garbage that the ellipse tool sometimes leaves on the
screen when moving around really fast.
M +1 -1 krita/ui/tool/kis_tool_ellipse_base.cpp
M +0 -1 krita/ui/tool/kis_tool_ellipse_base.h