From f4b1bb7bdf65cc323a69ea71dbe18578f8fdfe8c Mon Sep 17 00:00:00 2001
From: Matthew Stanley <1379tech@gmail.com>
Date: Mon, 26 Oct 2026 08:43:27 -0700
Subject: [PATCH] Reset backdrop stretch when batching supersampled world lines

Keep line quads separate from pending backdrop batches and reset the key after earlier batches flush. Exercise both draw orders with actual GL readback. Fixes the remaining Tomba 2 fishing string offset (beads-w0mm).
---
 runtime/src/gpu_gl_renderer.c            | 5 ++++-
 runtime/tests/test_gl_scale_invariance.c | 7 ++++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/runtime/src/gpu_gl_renderer.c b/runtime/src/gpu_gl_renderer.c
index e1442922..44223c83 100644
--- a/runtime/src/gpu_gl_renderer.c
+++ b/runtime/src/gpu_gl_renderer.c
@@ -4709,11 +4709,14 @@ static void gpu_geometry(GLenum mode, const int *xs, const int *ys,
         }
         line_to_quad(lv, quad);
         if (s_fb_n > 0 && (s_fb_mode != GL_TRIANGLES || s_fb_semi != semi ||
-                           s_fb_mask != (int)s_mask_set || s_fb_depth != 0))
+                           s_fb_mask != (int)s_mask_set || s_fb_gate != 0 || s_fb_depth != 0))
             flush_flat_batch();
         if (s_fb_n + 6 > FLATBATCH_MAXV)
             flush_flat_batch();
         s_fb_mode = GL_TRIANGLES;
+        /* The expanded line is still world geometry. A previous backdrop
+         * batch may have left its stretch key set even after being flushed. */
+        s_fb_gate = 0;
         s_fb_semi = semi;
         s_fb_mask = (int)s_mask_set;
         s_fb_depth = 0;   /* lines are 2D: never depth-tested (G1.14) */
diff --git a/runtime/tests/test_gl_scale_invariance.c b/runtime/tests/test_gl_scale_invariance.c
index 828242a4..9716c348 100644
--- a/runtime/tests/test_gl_scale_invariance.c
+++ b/runtime/tests/test_gl_scale_invariance.c
@@ -1298,10 +1298,15 @@ static int backdrop_lines_main(int scale) {
     g_ws_bd_stretch_on = 1;
     g_ws_bd_phase_mode = 1;
     g_ws_bd_stretch_pct = 0;
-    for (int policy = 0; policy < 2; ++policy) {
+    for (int policy = 0; policy < 2; ++policy) for (int order = 0; order < 3; ++order) {
         glb_wide_clear(0, 0, 512, 0);
         test_flat_backdrop = policy == 0;
         test_backdrop_phase = policy == 1;
+        /* Cover both a pending backdrop batch and its key remaining after a
+         * flush. Supersampled lines expand to triangles, but must not join
+         * that backdrop batch or inherit its horizontal stretch. */
+        if (order) glb_draw_flat_triangle(4, 4, 20, 4, 4, 12, 0x4210);
+        if (order == 2) gl_renderer_sync_cpu();
         glb_draw_line(64, 20, 64, 100, 0x7fff);
         glb_draw_shaded_line(100, 20, 0x7fff, 100, 100, 0x7fff);
         test_backdrop_phase = 0;