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;