Fix widescreen letterbox/divider left-edge clipping on ARM64 (#709)

* Fix widescreen letterbox/divider left-edge clipping on ARM64

The race-intro letterbox bars (draw_box_fill_wide) and the splitscreen
divider lines fed the float OTRGetDimensionFromLeftEdge result into
gDPFillWideRectangle, whose _SHIFTL packing casts float->unsigned.
Converting a negative float to unsigned is UB: x86 wraps (and the
wide-rect handler's sign extension recovers the value, so the bug is
invisible there), but ARM64 fcvtzu saturates negatives to zero — so on
Apple Silicon the fills started at the 4:3 left edge instead of the
true left edge, leaving a strip of sky visible left of the intro bars
in widescreen. Use the integer OTRGetRectDimensionFrom*Edge getters,
matching the sibling draw_box_wide helpers.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Shorten the fix comment

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

2 files changed, 12 insertions, 4 deletions

diff --git a/src/menu_items.c b/src/menu_items.c
index da6df9668..b6142ca37 100644
--- a/src/menu_items.c
+++ b/src/menu_items.c
@@ -3476,8 +3476,11 @@ Gfx* draw_box_fill_wide(Gfx* displayListHead, s32 ulx, s32 uly, s32 lrx, s32 lry
     gSPDisplayList(displayListHead++, D_02008030);
     gDPSetFillColor(displayListHead++, (GPACK_RGBA5551(red, green, (u32) blue, alpha) << 0x10 |
                                         GPACK_RGBA5551(red, green, (u32) blue, alpha)));
-    gDPFillWideRectangle(displayListHead++, OTRGetDimensionFromLeftEdge(ulx) - 1, uly, OTRGetDimensionFromRightEdge(lrx) + 1,
-                         lry);
+    // Use the integer Rect getters: the float getters' negative left edge goes
+    // through a float->unsigned cast in _SHIFTL (UB) that ARM64 saturates to 0,
+    // pushing the fill's left edge to the 4:3 boundary in widescreen.
+    gDPFillWideRectangle(displayListHead++, OTRGetRectDimensionFromLeftEdge(ulx) - 1, uly,
+                         OTRGetRectDimensionFromRightEdge(lrx) + 1, lry);
     gDPFillRectangle(displayListHead++, ulx, uly, lrx, lry);
     gSPDisplayList(displayListHead++, D_02008058);
     return displayListHead;
diff --git a/src/racing/skybox_and_splitscreen.c b/src/racing/skybox_and_splitscreen.c
index 825150f83..dff50e117 100644
--- a/src/racing/skybox_and_splitscreen.c
+++ b/src/racing/skybox_and_splitscreen.c
@@ -308,11 +308,16 @@ void func_802A4300(void) {
             gDPFillRectangle(gDisplayListHead++, 157, 0, 159, 239);
             break;
         case SCREEN_MODE_2P_SPLITSCREEN_HORIZONTAL:
-            gDPFillWideRectangle(gDisplayListHead++, OTRGetDimensionFromLeftEdge(0), 119, OTRGetGameRenderWidth(), 121);
+            // Integer Rect getter: the float getter's negative result through
+            // _SHIFTL is float->unsigned UB, which saturates to 0 on ARM64 and
+            // clips the divider's left extension (see draw_box_fill_wide).
+            gDPFillWideRectangle(gDisplayListHead++, OTRGetRectDimensionFromLeftEdge(0), 119,
+                                 OTRGetGameRenderWidth(), 121);
             break;
         case SCREEN_MODE_3P_4P_SPLITSCREEN:
             gDPFillRectangle(gDisplayListHead++, 157, 0, 159, 239);
-            gDPFillWideRectangle(gDisplayListHead++, OTRGetDimensionFromLeftEdge(0), 119, OTRGetGameRenderWidth(), 121);
+            gDPFillWideRectangle(gDisplayListHead++, OTRGetRectDimensionFromLeftEdge(0), 119,
+                                 OTRGetGameRenderWidth(), 121);
             break;
     }
     gDPPipeSync(gDisplayListHead++);