This is an issue with the following sequence of events:

SoftwareOutputDeviceMac::BeginPaint
- copies the front buffer to the back buffer
SoftwareRenderer::ClearFramebuffer
- clears the back buffer
SoftwareOutputDeviceMac::EndPaint
- swaps the front & back buffer

Two issues here
- the clear should happen before the copy
- the copy should be for the full window if we're going to do a clear

Hmm, this was on my bugs-to-look-at-when-I-have-time list, and ... I can't repro this anymore. Trying again.

I think this is fixed. In particular, the call sequence is
- DirectRenderer::DrawRenderPass
  - sets mode to SURFACE_INITIALIZATION_MODE_SCISSORED_CLEAR
- SoftwareRenderer::PrepareSurfaceForPass
  - sets the damage rect in SoftwareRenderer::SetScissorTestRect
- SoftwareRenderer::ClearFramebuffer
  - only clears the scissored rect

I'm pretty sure this was fixed by r477453 (thanks!).