Build a wgpu rendering backend for webrender. The GL backend is the reference — it works and produces correct output. We need the wgpu path to produce identical visual results.

At DPR=1 (no HiDPI), the wgpu pipeline renders text correctly. This proves the entire pipeline works: WGSL shaders, data texture uploads, ortho projection, tile rendering, composite-to-surface.

At DPR=2 (HiDPI display), text appeared 2x the correct CSS size and body text rendered teal instead of dark gray. Both symptoms had the same root cause.

File: webrender/src/renderer/mod.rs, function batch_key_to_pipeline_key()

The wgpu path hardcoded all text batches to use "ALPHA_PASS,TEXTURE_2D":

BatchKind::TextRun(..) => ("ps_text_run", "ALPHA_PASS,TEXTURE_2D"),

This ignored the GlyphFormat discriminant carried by BatchKind::TextRun(GlyphFormat). The GL path (in shade.rs:519-527) correctly selects between two shader variants:

• GlyphFormat::Alpha | Subpixel | Bitmap | ColorBitmap → simple (no glyph transform)
• GlyphFormat::TransformedAlpha | TransformedSubpixel → glyph_transform (with GLYPH_TRANSFORM feature)

Servo handles HiDPI by pushing a 2x reference frame transform (painter.rs:688-695) rather than setting global_device_pixel_scale. WebRender's global_device_pixel_scale is hardcoded to 1.0 (frame_builder.rs:684).

When a non-trivial transform is present (like 2x scale), WebRender's text run preparation (prim_store/text_run.rs:303-317) detects it's not a simple 2D translation and:

1. Sets transform_glyphs = true
2. Bakes the 2x transform into FontTransform → glyphs rasterized at 2x pixel size
3. Sets raster_scale = 1.0
4. Emits GlyphFormat::TransformedAlpha (or TransformedSubpixel)

The GLYPH_TRANSFORM shader variant handles this correctly:

// GLYPH_TRANSFORM path (correct):
mat2 glyph_transform = mat2(transform.m) * task.device_pixel_scale;
mat2 glyph_transform_inv = inverse(glyph_transform);
// ... compute glyph rect in glyph space, then transform back to local space
RectWithEndpoint local_rect = transform_rect(glyph_rect, glyph_transform_inv);

The non-transform path (what wgpu was using) does NOT compensate:

// Non-GLYPH_TRANSFORM path (wrong for transformed glyphs):
float glyph_scale_inv = res.scale / glyph_raster_scale;  // = 1.0
// glyph rect in local space = glyph_scale_inv * glyph_pixel_size
// → 1.0 * 46 texels = 46 CSS units → 92 device pixels after 2x transform (2x too big!)

The GLYPH_TRANSFORM and non-transform shader variants have different v_color and v_mask_swizzle computation in both vertex and fragment shaders. Using the wrong variant meant the color/mask logic was also incorrect, producing the teal-instead-of-gray symptom.

BatchKind::TextRun(glyph_format) => {
    match glyph_format {
        GlyphFormat::TransformedAlpha |
        GlyphFormat::TransformedSubpixel => {
            ("ps_text_run", "ALPHA_PASS,GLYPH_TRANSFORM,TEXTURE_2D")
        }
        _ => ("ps_text_run", "ALPHA_PASS,TEXTURE_2D"),
    }
}

At DPR=1, there is no 2x reference frame. The transform is identity (a simple 2D translation), so transform_glyphs = false, glyph format is Alpha (not Transformed*), and the non-transform shader variant is correct.

• Frame struct: same transforms, prim headers, render tasks, glyph atlas
• Tile cache DPS: 1.0 in both paths (surface at SpatialNodeIndex(0), same as root)
• World scale factors: (1.0, 1.0) in both paths
• GLSL vs WGSL shader math: identical (verified line-by-line via naga output)
• GPU cache addressing: identical (texelFetch/textureLoad, same UV math)
• Data texture layout: identical (MAX_VERTEX_TEXTURE_WIDTH=1024, same packing)
• Composite tile rects: identical placement on surface

surface size: 2048x1480 (physical pixels, DPR=2)
tile texture: 1024x512 (Rgba8Unorm)
transform[2]: row0=[2.000,0.000,0.000,0.000] row1=[0.000,2.000,0.000,0.000]
render_task[0]: data=[0.0, 0.0, 1024.0, 512.0, 1.0, 0.0, 0.0, 0.0]
  → task_rect=(0,0)-(1024,512), DPS=1.0, content_origin=(0,0)
user_data=[65535, 0, 0, 0] → raster_scale=1.0
DPS_DIAG: surface[0].device_pixel_scale=1.000, world_scale=(1.000,1.000)
WSF_DIAG: parent=None surface_node=SpatialNodeIndex(0) root_ref=SpatialNodeIndex(0) scale=(1.000,1.000)

# Before fix:
wgpu batch[1]: shader="ps_text_run" config="ALPHA_PASS,TEXTURE_2D" ← WRONG

# After fix:
wgpu batch[1]: shader="ps_text_run" config="ALPHA_PASS,GLYPH_TRANSFORM,TEXTURE_2D" ← CORRECT

1. Tile cache DPS should be 2.0 — No, DPS=1.0 is correct; GL uses same value
2. Data texture upload mismatch — No, CPU data identical, layout math identical
3. Ortho projection difference — No, both use tile texture dimensions
4. Composite rect/UV mapping — No, both use same get_device_rect()
5. GPU cache addressing — No, verified get_gpu_cache_uv identical in WGSL
6. Surface double-scaling — No, PerMonitorV2 DPI awareness is set

1. texels_per_item 3→2 for render tasks (mod.rs)
2. Clear color: draw_instanced signature → clear_color: Option<wgpu::Color>
3. Picture cache tiles: use picture_target.clear_color instead of hardcoded black
4. Tile projection: use full texture dimensions, not dirty_rect (mod.rs)
5. Text at DPR=2: Select GLYPH_TRANSFORM shader variant for TransformedAlpha/TransformedSubpixel glyph formats (mod.rs batch_key_to_pipeline_key)

• webrender/src/renderer/mod.rs — wgpu render path + GL render path (reference)
  • batch_key_to_pipeline_key() — the fix location
  • wgpu tile rendering: lines ~2000-2340
  • GL tile rendering: draw_picture_cache_target at line 5277
  • GL composite: composite_frame at line 7680, composite_simple at line 5969
  • wgpu composite: lines 1600-1800
• webrender/src/renderer/shade.rs:519-527 — GL shader selection (reference for fix)
• webrender/src/device/wgpu_device.rs — draw_instanced, composite pipeline, ortho()
• webrender/src/picture.rs — tile cache setup, DPS computation
• webrender/src/prim_store/text_run.rs:270-338 — font instance update, transform_glyphs logic
• webrender/src/frame_builder.rs:684 — global_device_pixel_scale = 1.0 (hardcoded)
• servo-graphshell/components/paint/painter.rs:688 — 2x HiDPI reference frame
• webrender/res/ps_text_run.glsl:130-198 — GLYPH_TRANSFORM vs non-transform shader paths