--- a/src/MacSrc/OpenGL.cc	2026-07-19 13:11:51.000000000 +0200
+++ b/src/MacSrc/OpenGL.cc	2026-09-27 11:42:22.497868697 +0200
@@ -562,7 +562,14 @@
 static bool opengl_cache_texture(CachedTexture toCache, grs_bitmap *bm) {
     SDL_GL_MakeCurrent(window, context);
 
-    glPixelStorei(GL_UNPACK_ROW_LENGTH, bm->row);
+    // GL_UNPACK_ROW_LENGTH must describe the buffer actually passed to
+    // glTexImage2D() below (toCache.converted->pixels, a separately
+    // allocated SDL_PIXELFORMAT_RGBA32 surface from
+    // SDL_ConvertSurfaceFormat()) - not bm->row, the original 8-bit source
+    // bitmap's own unrelated row stride. Using bm->row here describes the
+    // wrong stride to OpenGL whenever the two differ, corrupting the
+    // texture upload.
+    glPixelStorei(GL_UNPACK_ROW_LENGTH, toCache.converted->pitch / 4);
 
     if (texturesByBitsPtr.size() < MAX_CACHED_TEXTURES) {
         // We have enough room, generate the new texture
@@ -696,7 +703,10 @@
     t->lastDrawTime = *tmd_ticks;
 
     if (isDirty) {
-        glPixelStorei(GL_UNPACK_ROW_LENGTH, bm->row);
+        // Same fix as opengl_cache_texture() above - t->converted->pixels is
+        // what's actually uploaded, so its own pitch is what
+        // GL_UNPACK_ROW_LENGTH needs to describe, not bm->row.
+        glPixelStorei(GL_UNPACK_ROW_LENGTH, t->converted->pitch / 4);
         glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, bm->w, bm->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, t->converted->pixels);
     }
 }