Also proposed on Hookshot: simonwjackson/Hookshot feat-window-aspect.
Cherry-picked source: e7481e2, 4813a27, 92b3eb5, a71778d, d1d9cdc.
Resolve rename/context conflicts without changing feature behaviour.
Keep measured configurations in the PR description, not the shared table.

Assisted-by: OpenAI GPT-6 Astra
Co-Authored-By: OpenAI GPT-6 Astra <[email protected]>

@@ -497,6 +497,12 @@ HDR, Detail, and GPU and memory (last: the rows to change when something is wron
   and **Sharpness** sharpens it, by the picture's pixels: when the window is much larger than the picture (the Handheld
   Profile's 1536x864 on a 4K screen), a Sharpness kept from an older version is stronger than it was, so a lower one may
   suit.
+- **Window aspect ratio** (opt-in): fills the actual window. Wider than 16:9 shows more at the sides (Hor+);

@@ -3989,6 +4006,8 @@ void draw_startup(ui::Ctx& c) {
 
 } // namespace
 
+bool window_fit_active() { return ultrawide_read == 2 && !getenv("HOS_ASPECT"); }

@@ -0,0 +16,4 @@
+
+float hos_wide_aspect(void) { return hos_wide_window ? atomic_load_explicit(&window_aspect, memory_order_relaxed) : hos_aspect_ratio; }
+
+void hos_wide_resize(unsigned width, unsigned height) {

@@ -0,0 +21,4 @@
+    if (aspect < 16.0 / 9.0) oh = (unsigned)(ow / aspect / 8 + 0.5) * 8;
+    else ow = (unsigned)(oh * aspect / 8 + 0.5) * 8;
+    /* Upstream's render-target pool guard (issue 1031): cap pixels, not shape. */
+    if ((double)ow * oh > hos_wide_max_px()) {

@@ -300,1 +312,4 @@
     if (getenv("HOS_FULLSCREEN") && atoi(getenv("HOS_FULLSCREEN"))) set_fullscreen(1);
+    int pixel_w = 0, pixel_h = 0;
+    hos_sdl_pixel_size(window, &pixel_w, &pixel_h);
+    hos_wide_start(pixel_w, pixel_h);

@@ -51,1 +87,4 @@
+
+HUD = "{ extern int hos_wide_window; if (hos_wide_window) " + HUD + " else " + HUD_FIXED + " }"
+
 MOVIE = ("{ extern volatile float hos_wide_margin; const float _m = hos_wide_margin; if (_m > 0.0f) { "

@@ -48,6 +72,21 @@ HUD = ("{ extern volatile float hos_wide_squeeze; const float _q = hos_wide_sque
        "memcpy(c->vreg[1], _row, 16); } } } }   /* (nx2/oot_wide.py: HUD in the middle 16:9) */"
        % (UI_M00, UI_M11, " : ".join("_vt == 0x%xULL ? %du" % (v, 1 << k) for k, v in enumerate(KEEP_VTABLES)) + " : 0u"))
 
+HUD = ("{ extern float hos_wide_aspect(void); const float _w = hos_wide_aspect(); if (_w > 0.0f) { float _m00, _m11; "