I want to triage a bug that bothers me, where the brush circle (cursor) disappears.

Can someone point me to the code that draws the circle (cursor) of a brush?

Thanks,

That depends. There are three places where cursors are drawn: the real cursor is drawn by the OS. The brush outline can be drawn in opengl or on the CPU. If canvas accceleration is enabled, the outline drawing code is in KisOpenGLCanvas2::paintToolOutline