gpui: Store GPU-facing bools as PaddedBool32 to avoid uninitialized padding (#60482)

`PolychromeSprite` in `crates/gpui/src/scene.rs` is `#[repr(C)]` and had
a `grayscale: bool` field followed by 3 compiler-inserted padding bytes
that were never written. The wgpu renderer's `instance_bytes`
reinterprets `&[PolychromeSprite]` as `&[u8]` via
`slice::from_raw_parts` and passes it to `queue.write_buffer`, so those
uninitialized padding bytes were exposed behind a shared `&[u8]` on
every frame that draws an image or emoji, which is undefined behavior.

Rather than widening the field to a raw `u32` (which would suggest
values other than 0 and 1 are meaningful), this introduces
`PaddedBool32`: a `#[repr(transparent)]` wrapper around `u32` whose only
public constructor is `From<bool>`, so the 0-or-1 invariant is enforced
by the type while the layout has no padding. `Underline.wavy`, which was
already a raw `u32` for the same reason, is converted too.

cbindgen emits the wrapper as `typedef uint32_t PaddedBool32;`, so the
generated Metal header and shaders are unchanged. The WGSL and HLSL
shaders already declared these fields as `u32`/`uint`; their `& 0xFFu`
masks, which existed to ignore the garbage padding bytes, are now
simplified to plain comparisons.

Release Notes:

- N/A

crates/gpui/src/scene.rs

@@ -22,6 +22,20 @@ pub type PathVertex_ScaledPixels = PathVertex<ScaledPixels>;
 #[expect(missing_docs)]
 pub type DrawOrder = u32;
 
+/// A boolean stored as a `u32` so that GPU-facing structs contain no
+/// compiler-inserted padding bytes, which would be undefined behavior to
+/// reinterpret as `&[u8]` when writing instance buffers. Guaranteed to be
+/// `0` or `1` by construction; shaders read it as a `u32`/`uint`.
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq)]
+#[repr(transparent)]
+pub struct PaddedBool32(u32);
+
+impl From<bool> for PaddedBool32 {
+    fn from(value: bool) -> Self {
+        PaddedBool32(value as u32)
+    }
+}
+
 #[derive(Default)]
 #[expect(missing_docs)]
 pub struct Scene {
@@ -511,7 +525,7 @@ pub struct Underline {
     pub content_mask: ContentMask<ScaledPixels>,
     pub color: Hsla,
     pub thickness: ScaledPixels,
-    pub wavy: u32,
+    pub wavy: PaddedBool32,
 }
 
 impl From<Underline> for Primitive {
@@ -701,7 +715,7 @@ impl From<SubpixelSprite> for Primitive {
 pub struct PolychromeSprite {
     pub order: DrawOrder,
     pub pad: u32,
-    pub grayscale: bool,
+    pub grayscale: PaddedBool32,
     pub opacity: f32,
     pub bounds: Bounds<ScaledPixels>,
     pub content_mask: ContentMask<ScaledPixels>,

crates/gpui/src/window.rs

@@ -3812,7 +3812,7 @@ impl Window {
             content_mask: self.snapped_content_mask(),
             color: style.color.unwrap_or_default().opacity(element_opacity),
             thickness,
-            wavy: if style.wavy { 1 } else { 0 },
+            wavy: style.wavy.into(),
         });
     }
 
@@ -3842,7 +3842,7 @@ impl Window {
             content_mask: self.snapped_content_mask(),
             thickness: self.snap_stroke(style.thickness),
             color: style.color.unwrap_or_default().opacity(opacity),
-            wavy: 0,
+            wavy: false.into(),
         });
     }
 
@@ -4002,7 +4002,7 @@ impl Window {
             self.next_frame.scene.insert_primitive(PolychromeSprite {
                 order: 0,
                 pad: 0,
-                grayscale: false,
+                grayscale: false.into(),
                 bounds,
                 corner_radii: Default::default(),
                 content_mask,
@@ -4117,7 +4117,7 @@ impl Window {
         self.next_frame.scene.insert_primitive(PolychromeSprite {
             order: 0,
             pad: 0,
-            grayscale,
+            grayscale: grayscale.into(),
             bounds,
             content_mask,
             corner_radii,

crates/gpui_wgpu/src/shaders.wgsl

@@ -1191,7 +1191,7 @@ fn fs_underline(input: UnderlineVarying) -> @location(0) vec4<f32> {
     }
 
     let underline = b_underlines[input.underline_id];
-    if ((underline.wavy & 0xFFu) == 0u)
+    if (underline.wavy == 0u)
     {
         return blend_color(input.color, input.color.a);
     }
@@ -1305,7 +1305,7 @@ fn fs_poly_sprite(input: PolySpriteVarying) -> @location(0) vec4<f32> {
     let distance = quad_sdf(input.position.xy, sprite.bounds, sprite.corner_radii);
 
     var color = sample;
-    if ((sprite.grayscale & 0xFFu) != 0u) {
+    if (sprite.grayscale != 0u) {
         let grayscale = dot(color.rgb, GRAYSCALE_FACTORS);
         color = vec4<f32>(vec3<f32>(grayscale), sample.a);
     }

crates/gpui_windows/src/shaders.hlsl

@@ -1249,7 +1249,7 @@ float4 polychrome_sprite_fragment(PolychromeSpriteFragmentInput input): SV_Targe
     float distance = quad_sdf(input.position.xy, sprite.bounds, sprite.corner_radii);
 
     float4 color = sample;
-    if ((sprite.grayscale & 0xFFu) != 0u) {
+    if (sprite.grayscale != 0u) {
         float3 grayscale = dot(color.rgb, GRAYSCALE_FACTORS);
         color = float4(grayscale, sample.a);
     }