A Xenos RectangleList gives 3 corners of a rectangle; the GPU synthesizes
the 4th (v3 = v0 + v2 - v1) and tessellates the quad as two triangles
(v0,v1,v2) + (v0,v2,v3). Our expansion emitted only the front triangle, so
the movie's fullscreen YUV rect filled half the screen and left a diagonal
seam down the frame (absent in canary).
expand_rectangles now emits the full 6-index quad. Since v3 has no backing
vertex in the guest window, the translated VS synthesizes its attributes:
the RectangleList arm maps 6 host verts through [0,1,2, 0,2,3], flags the
4th corner, and emit_vfetch computes that corner's attributes (position and
every interpolator) as v0 + v2 - v1 by reading the three real source
vertices. Non-rect draws are unaffected (the synth flag is false).
Verified: the movie background now fills the whole frame uniformly, no
diagonal, matching canary.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
deb9292395