Preserve background composite lifetime and park visual validation

docs/WIDESCREEN.md

@@ -18,12 +18,27 @@ the composite remains continuous. Tags expire after two frames and reject
 reused packets whose words changed. This method performs no guest writes or
 extra guest allocations.
 
-OpenGL preserves the tag across flat and textured batching. While an explicitly
-tagged background is active, it renders the full wide composite instead of
-copying the canonical center over it, which would create a scale seam. Other
-scenes retain the center-copy optimization. Measure performance in the title
-and validate the visible sky at the requested aspect before release. Vulkan's
-native-wide compositor is not implemented; this API does not add one.
+OpenGL preserves the tag across flat and textured batching. Once a valid
+background has been tagged, it renders the full wide composite instead of
+copying the canonical center over it, which would create a scale seam. Packet
+freshness and displayed-pixel lifetime are separate: a slow game or held display
+can retain stretched pixels after the two-frame packet guard expires. The
+full-composite requirement therefore remains latched until GPU reset or snapshot
+restore; packet-address/content freshness is not relaxed. Sessions that have not
+tagged a background retain the center-copy optimization. Measure performance in
+the title and validate the visible sky at the requested aspect before release.
+Vulkan's native-wide compositor is not implemented; this API does not add one.
+
+### Parked validation checkpoint (2026-09-12)
+
+The focused GPU packet/tag regression passes, including expired packets,
+changed packet contents, disabled widescreen and retained composite lifetime.
+The V8 Windows runtime builds and both game CTests pass. In the oil-field
+21:9 playtest the owner reports that terrain looks fine and the background
+flickers less badly, but still flickers. This is partial improvement, not visual
+acceptance. State replay required the HLE scheduler; repeat the comparison with
+matched scheduler settings before attributing the improvement solely to this
+change. Work is parked in a draft; no release is approved.
 
 ## Earlier projection-and-stretch implementation
 

runtime/include/gpu.h

@@ -366,9 +366,12 @@ void gpu_ws_tag_hud_prim(uint32_t prim, int anchor);
  * Tag the complete background composite before submitting its ordering table.
  * The caller proves background identity/coverage; no guest data is modified. */
 void gpu_ws_tag_background_prim(uint32_t prim);
-/* Renderer policy: tagged background scenes need their full wide composite;
- * copying the canonical center over it would introduce a scale discontinuity. */
+/* Fresh packet tags are eligible for background classification. */
 int gpu_ws_background_stretch_active(void);
+/* A tagged background can remain in a wide surface after its packet expires.
+ * Once used, retain full compositing until GPU reset or snapshot restoration.
+ * Packet freshness must never re-enable canonical-center overwrites. */
+int gpu_ws_background_requires_full_composite(void);
 /* Clear synthetic margins to black over an opaque 0x64/65 rectangle's Y band
  * immediately before it executes. Packet-guarded; canonical VRAM is untouched. */
 void gpu_ws_tag_black_reveal_rect(uint32_t prim);

runtime/src/gpu.c

@@ -1967,6 +1967,15 @@ int gpu_ws_background_stretch_active(void) {
            frame - ws_background_tag_frame <= WS_HUD_ANCHOR_FRESH_FRAMES;
 }
 
+int gpu_ws_background_requires_full_composite(void) {
+    /* Freshness governs reuse of a packet address, not the lifetime of the
+     * pixels it produced. A slow game frame or a held display buffer can keep
+     * those pixels on screen indefinitely. Conservatively keep full mirroring
+     * once a composite has been tagged; GPU reset/snapshot restore clears the
+     * latch together with the tags. Never modify the packet freshness guard. */
+    return ws_background_tags_used && ws_native_wide_configured();
+}
+
 static int ws_nw_explicit_background(void) {
     if (!gpu_ws_background_stretch_active() ||
         gp0_cmd_source_addr == 0xFFFFFFFFu ||

runtime/src/gpu_gl_renderer.c

@@ -1705,7 +1705,7 @@ static int wide_fast_center_valid(void) {
     /* An explicitly stretched sky differs inside the canonical viewport too.
      * Keep the full composite for those scenes, including later foreground
      * draws. Tags are installed before DMA, so no earlier center draws skip. */
-    return s_wide_fast && !gpu_ws_background_stretch_active();
+    return s_wide_fast && !gpu_ws_background_requires_full_composite();
 }
 static void wide_blit_center(GLuint wide_fbo, int base_x, int disp_y, int disp_h); /* def below */
 /* True if [lo,hi] (canonical draw-x) lies strictly inside the 4:3 frame, so the

runtime/tests/test_gpu_textured_dot_nw_shift_exec.c

@@ -113,11 +113,13 @@ static void test_background_packet_tags(void) {
     memcpy(gp0_cmd_buf, packet, sizeof(packet));
     gpu_ws_tag_background_prim(addr - 4u);
     assert(!gpu_ws_background_stretch_active());
+    assert(!gpu_ws_background_requires_full_composite());
     assert(!ws_nw_explicit_background());
 
     configure_native_wide_16_9();
     gpu_ws_tag_background_prim(0x80000000u | (addr - 4u));
     assert(gpu_ws_background_stretch_active());
+    assert(gpu_ws_background_requires_full_composite());
     assert(ws_nw_explicit_background());
     /* Explicit proof remains valid after a foreground phase has begun. */
     s_bg_phase_frame = (uint32_t)s_frame_count;
@@ -141,11 +143,19 @@ static void test_background_packet_tags(void) {
     ++s_frame_count;
     assert(!gpu_ws_background_stretch_active());
     assert(!ws_nw_explicit_background());
+    /* A held display retains its stretched pixels after packet expiry. The
+     * compositor must not paste canonical 4:3 scenery over their center. */
+    assert(gpu_ws_background_requires_full_composite());
+    s_frame_count += 120;
+    assert(gpu_ws_background_requires_full_composite());
+    assert(!ws_nw_explicit_background());
     s_frame_count = 99;
     assert(!gpu_ws_background_stretch_active());
+    assert(gpu_ws_background_requires_full_composite());
     s_frame_count = 100;
     ws_mode = 0;
     assert(!gpu_ws_background_stretch_active());
+    assert(!gpu_ws_background_requires_full_composite());
 
     /* Reject semi-transparent polygons, rectangles, misalignment and RAM
      * overflow; a rejected tag must not even enable the full-composite path. */
@@ -153,6 +163,7 @@ static void test_background_packet_tags(void) {
                                  0x1F800000u};
     reset_gpu_state_for_test();
     configure_native_wide_16_9();
+    assert(!gpu_ws_background_requires_full_composite());
     for (unsigned i = 0; i < sizeof(bad_prim) / sizeof(bad_prim[0]); ++i)
         gpu_ws_tag_background_prim(bad_prim[i]);
     assert(!gpu_ws_background_stretch_active());