--- a/src/MacSrc/OpenGL.cc
+++ b/src/MacSrc/OpenGL.cc
@@ -566,7 +566,11 @@
     glDisableVertexAttribArray(0);
     glDisableVertexAttribArray(tcAttrib);
     glDisableVertexAttribArray(lightAttrib);
-    glFlush();
+    // glFinish(), not glFlush(): xr_swapchain_release() (called by our
+    // caller right after this returns) hands the image to the OpenXR
+    // runtime for cross-process consumption, which requires the GPU write
+    // to have actually landed, not just be queued.
+    glFinish();
 
     SDL_FreeSurface(uiRgba);
 }