perf(terminal): draw webgl backdrops with quads

TERMINAL_ROADMAP.md

@@ -146,7 +146,7 @@
    - WebGL2 初始化 shader/program/vertex buffer/atlas texture/fallback texture。
    - `GlyphAtlas` 按 grapheme + bold/italic + cell width 缓存字形；ASCII、CJK wide 字符、组合字符按 `unicode-segmentation` / `unicode-width` 拆成 cell glyph。
    - 文本通过 per-cell quad 批量绘制，颜色作为 vertex attribute；小字号不再走整屏纹理二次采样。
-   - 背景、underline、cursor 背景由 Canvas backdrop 绘制后作为底层 texture 合成；cursor 前景文字仍进 atlas。
+   - 背景、underline、cursor 背景在 WebGL 正常路径中直接绘制为纯色 quad；cursor 前景文字仍进 atlas。
    - atlas 溢出或遇到不可拆分复杂段时自动回退整屏 Canvas texture，避免真实终端内容白屏。
    - WebGL2 不可用时自动落回 Canvas 2D。
 2. **Canvas 2D 渲染器** ✅
@@ -255,6 +255,10 @@
    - WebGL render path 记录阶段耗时：DOM/canvas 同步、backdrop 绘制、glyph collect、backdrop texture pass、glyph atlas、atlas upload、vertex build/upload、draw call。
    - 记录计数：cells、atlas entries、atlas insertions/resets、full-canvas fallback 次数、glyph draw failed 次数。
    - `window.__terminalBench.webglFrames` 暴露每帧采样，`scripts/terminal-bench.mjs` 汇总为 `rendererStats`，用于判断慢点是整屏 backdrop texture、atlas、vertex buffer，还是 fallback。
+6. **WebGL backdrop texture pass 拆除** ✅
+   - 正常 WebGL 路径不再每帧把 Canvas backdrop 上传为整屏 texture；背景、underline、cursor 背景改为 1×1 白色纹理 + 纯色 quad 批量绘制。
+   - `backdropTextureMs` 只在复杂 glyph / atlas 失败等 full-canvas fallback 路径产生，用于继续观察 fallback 是否被真实内容触发。
+   - 页面切换、`?renderer=webgl` 和 `--renderer webgl` 保留，下一轮基准可直接对比这次优化后 WebGL 是否追近 Canvas。
 
 ---
 

app/src/terminal/renderer.rs

@@ -460,6 +460,7 @@ pub(super) struct WebGlRendererState {
     program: WebGlProgram,
     atlas_texture: WebGlTexture,
     fallback_texture: WebGlTexture,
+    solid_texture: WebGlTexture,
     buffer: WebGlBuffer,
     position_location: u32,
     tex_coord_location: u32,
@@ -573,6 +574,15 @@ struct GlyphQuad {
     color: [f32; 4],
 }
 
+#[derive(Clone, Copy, Debug, PartialEq)]
+struct SolidQuad {
+    x: f64,
+    y: f64,
+    width: f64,
+    height: f64,
+    color: [f32; 4],
+}
+
 fn split_segment_glyphs(
     segment: &TerminalSegment,
     row: usize,
@@ -631,6 +641,75 @@ fn collect_glyph_cells(
     Some(cells)
 }
 
+fn collect_backdrop_quads(frame: CanvasSurfaceFrame<'_>) -> Option<Vec<SolidQuad>> {
+    let (cell_w, cell_h) = frame.cell_size;
+    let mut quads = Vec::new();
+
+    for (row_idx, row) in frame.visible.iter().enumerate() {
+        let y = frame.content_top_px + row_idx as f64 * cell_h;
+        let mut col = 0usize;
+        for segment in &row.segments {
+            let x = col as f64 * cell_w;
+            let width = segment.cols as f64 * cell_w;
+            if segment.is_cursor {
+                let rect =
+                    cursor_shape_rect(frame.options.cursor.style, x, y, width.max(cell_w), cell_h);
+                push_color_rect(
+                    &mut quads,
+                    rect.x,
+                    rect.y,
+                    rect.width,
+                    rect.height,
+                    &frame.options.theme.cursor_background,
+                )?;
+            } else {
+                push_color_rect(
+                    &mut quads,
+                    x,
+                    y,
+                    width,
+                    cell_h,
+                    themed_color(&segment.style.background, frame.options),
+                )?;
+            }
+
+            if segment.style.underline {
+                let color = if segment.is_cursor {
+                    &frame.options.theme.cursor_foreground
+                } else {
+                    themed_color(&segment.style.color, frame.options)
+                };
+                push_color_rect(&mut quads, x, y + cell_h - 1.0, width, 1.0, color)?;
+            }
+            col += segment.cols;
+        }
+    }
+
+    Some(quads)
+}
+
+fn push_color_rect(
+    quads: &mut Vec<SolidQuad>,
+    x: f64,
+    y: f64,
+    width: f64,
+    height: f64,
+    color: &str,
+) -> Option<()> {
+    let color = parse_css_rgb(color)?;
+    if color[3] <= 0.0 || width <= 0.0 || height <= 0.0 {
+        return Some(());
+    }
+    quads.push(SolidQuad {
+        x,
+        y,
+        width,
+        height,
+        color,
+    });
+    Some(())
+}
+
 fn parse_css_rgb(color: &str) -> Option<[f32; 4]> {
     let color = color.trim();
     if color == "transparent" {
@@ -678,6 +757,21 @@ fn push_quad(vertices: &mut Vec<f32>, quad: GlyphQuad, dom_state: CanvasDomState
     vertices.extend_from_slice(&data);
 }
 
+fn push_solid_quad(vertices: &mut Vec<f32>, quad: SolidQuad, dom_state: CanvasDomState) {
+    push_quad(
+        vertices,
+        GlyphQuad {
+            x: quad.x,
+            y: quad.y,
+            width: quad.width,
+            height: quad.height,
+            uv: [0.0, 0.0, 1.0, 1.0],
+            color: quad.color,
+        },
+        dom_state,
+    );
+}
+
 fn glyph_uv(entry: GlyphAtlasEntry, atlas_size: u32) -> [f64; 4] {
     let size = f64::from(atlas_size);
     [
@@ -716,47 +810,53 @@ pub(super) fn render_webgl(
         return Err("WebGL context is lost".to_owned());
     }
 
-    let backdrop_start = perf_now();
-    sync_canvas_bitmap(&state.source_canvas, frame.dom_state);
-    let _ = state
-        .source_ctx
-        .set_transform(frame.dpr, 0.0, 0.0, frame.dpr, 0.0, 0.0);
-    render_canvas_backdrop(&state.source_ctx, frame);
-    stats.backdrop_ms = perf_now() - backdrop_start;
-
     let collect_start = perf_now();
     let Some(cells) = collect_glyph_cells(frame.visible, frame.options) else {
         stats.collect_ms = perf_now() - collect_start;
-        stats.fallback_full_canvas = true;
+        render_webgl_canvas_fallback(state, frame, &mut stats)?;
-        let fallback_canvas_start = perf_now();
+        stats.total_ms = perf_now() - total_start;
-        render_canvas(&state.source_ctx, frame);
+        return Ok(stats);
-        stats.fallback_canvas_ms += perf_now() - fallback_canvas_start;
+    };
-        let texture_start = perf_now();
+    let Some(backdrop_quads) = collect_backdrop_quads(frame) else {
-        draw_webgl_texture_frame(state, frame.dom_state)?;
+        stats.collect_ms = perf_now() - collect_start;
-        stats.backdrop_texture_ms += perf_now() - texture_start;
+        render_webgl_canvas_fallback(state, frame, &mut stats)?;
         stats.total_ms = perf_now() - total_start;
         return Ok(stats);
     };
     stats.collect_ms = perf_now() - collect_start;
     stats.cells = cells.len();
 
-    let texture_start = perf_now();
+    let backdrop_start = perf_now();
-    draw_webgl_texture_frame(state, frame.dom_state)?;
+    draw_webgl_backdrop_quads(state, frame.dom_state, &backdrop_quads)?;
-    stats.backdrop_texture_ms += perf_now() - texture_start;
+    stats.backdrop_ms = perf_now() - backdrop_start;
     if draw_webgl_glyphs(state, frame, &cells, &mut stats).is_err() {
         stats.glyph_draw_failed = true;
-        let fallback_canvas_start = perf_now();
+        render_webgl_canvas_fallback(state, frame, &mut stats)?;
-        render_canvas(&state.source_ctx, frame);
-        stats.fallback_canvas_ms += perf_now() - fallback_canvas_start;
-        let texture_start = perf_now();
-        draw_webgl_texture_frame(state, frame.dom_state)?;
-        stats.backdrop_texture_ms += perf_now() - texture_start;
     }
     stats.atlas_entries = state.atlas.entries.len();
     stats.total_ms = perf_now() - total_start;
     Ok(stats)
 }
 
+fn render_webgl_canvas_fallback(
+    state: &WebGlRendererState,
+    frame: CanvasSurfaceFrame<'_>,
+    stats: &mut WebGlRenderStats,
+) -> Result<(), String> {
+    stats.fallback_full_canvas = true;
+    let fallback_canvas_start = perf_now();
+    sync_canvas_bitmap(&state.source_canvas, frame.dom_state);
+    let _ = state
+        .source_ctx
+        .set_transform(frame.dpr, 0.0, 0.0, frame.dpr, 0.0, 0.0);
+    render_canvas(&state.source_ctx, frame);
+    stats.fallback_canvas_ms += perf_now() - fallback_canvas_start;
+    let texture_start = perf_now();
+    draw_webgl_texture_frame(state, frame.dom_state)?;
+    stats.backdrop_texture_ms += perf_now() - texture_start;
+    Ok(())
+}
+
 fn create_webgl_renderer(canvas: &HtmlCanvasElement) -> Result<WebGlRendererState, String> {
     let gl = canvas
         .get_context("webgl2")
@@ -805,6 +905,7 @@ fn create_webgl_renderer(canvas: &HtmlCanvasElement) -> Result<WebGlRendererStat
         .create_texture()
         .ok_or_else(|| "failed to create WebGL fallback texture".to_owned())?;
     configure_webgl_texture(&gl, &fallback_texture);
+    let solid_texture = create_webgl_solid_texture(&gl)?;
     gl.pixel_storei(WebGl2RenderingContext::UNPACK_FLIP_Y_WEBGL, 1);
     gl.pixel_storei(WebGl2RenderingContext::UNPACK_PREMULTIPLY_ALPHA_WEBGL, 1);
     gl.enable(WebGl2RenderingContext::BLEND);
@@ -851,6 +952,7 @@ fn create_webgl_renderer(canvas: &HtmlCanvasElement) -> Result<WebGlRendererStat
         program,
         atlas_texture,
         fallback_texture,
+        solid_texture,
         buffer,
         position_location: position_location as u32,
         tex_coord_location: tex_coord_location as u32,
@@ -883,6 +985,27 @@ fn configure_webgl_texture(gl: &WebGl2RenderingContext, texture: &WebGlTexture)
     );
 }
 
+fn create_webgl_solid_texture(gl: &WebGl2RenderingContext) -> Result<WebGlTexture, String> {
+    let texture = gl
+        .create_texture()
+        .ok_or_else(|| "failed to create WebGL solid texture".to_owned())?;
+    configure_webgl_texture(gl, &texture);
+    let white = [255, 255, 255, 255];
+    gl.tex_image_2d_with_i32_and_i32_and_i32_and_format_and_type_and_opt_u8_array(
+        WebGl2RenderingContext::TEXTURE_2D,
+        0,
+        WebGl2RenderingContext::RGBA as i32,
+        1,
+        1,
+        0,
+        WebGl2RenderingContext::RGBA,
+        WebGl2RenderingContext::UNSIGNED_BYTE,
+        Some(&white),
+    )
+    .map_err(|_| "failed to upload WebGL solid texture".to_owned())?;
+    Ok(texture)
+}
+
 fn compile_webgl_shader(
     gl: &WebGl2RenderingContext,
     shader_type: u32,
@@ -1013,6 +1136,46 @@ fn draw_webgl_texture_frame(
     Ok(())
 }
 
+fn draw_webgl_backdrop_quads(
+    state: &WebGlRendererState,
+    dom_state: CanvasDomState,
+    quads: &[SolidQuad],
+) -> Result<(), String> {
+    let gl = &state.gl;
+    gl.viewport(
+        0,
+        0,
+        dom_state.backing_width as i32,
+        dom_state.backing_height as i32,
+    );
+    gl.clear_color(0.0, 0.0, 0.0, 0.0);
+    gl.clear(WebGl2RenderingContext::COLOR_BUFFER_BIT);
+    if quads.is_empty() {
+        return Ok(());
+    }
+
+    let mut vertices = Vec::with_capacity(quads.len() * WEBGL_VERTEX_FLOATS * 6);
+    for quad in quads {
+        push_solid_quad(&mut vertices, *quad, dom_state);
+    }
+
+    gl.use_program(Some(&state.program));
+    bind_webgl_vertex_layout(gl, state);
+    upload_webgl_vertices(gl, &vertices);
+    gl.active_texture(WebGl2RenderingContext::TEXTURE0);
+    gl.bind_texture(
+        WebGl2RenderingContext::TEXTURE_2D,
+        Some(&state.solid_texture),
+    );
+    gl.uniform1i(state.sampler_location.as_ref(), 0);
+    gl.draw_arrays(
+        WebGl2RenderingContext::TRIANGLES,
+        0,
+        (vertices.len() / WEBGL_VERTEX_FLOATS) as i32,
+    );
+    Ok(())
+}
+
 fn draw_webgl_glyphs(
     state: &mut WebGlRendererState,
     frame: CanvasSurfaceFrame<'_>,
@@ -1174,39 +1337,6 @@ fn draw_glyph_into_atlas(
     Ok(Some(entry))
 }
 
-fn render_canvas_backdrop(ctx: &CanvasRenderingContext2d, frame: CanvasSurfaceFrame<'_>) {
-    let (cell_w, cell_h) = frame.cell_size;
-    let canvas_width = ctx.canvas().map(|c| c.width() as f64).unwrap_or(0.0);
-    let canvas_height = ctx.canvas().map(|c| c.height() as f64).unwrap_or(0.0);
-    ctx.clear_rect(0.0, 0.0, canvas_width, canvas_height);
-
-    for (row_idx, row) in frame.visible.iter().enumerate() {
-        let y = frame.content_top_px + row_idx as f64 * cell_h;
-        let mut col = 0usize;
-        for segment in &row.segments {
-            let x = col as f64 * cell_w;
-            let width = segment.cols as f64 * cell_w;
-            if segment.is_cursor {
-                ctx.set_fill_style_str(&frame.options.theme.cursor_background);
-                let rect =
-                    cursor_shape_rect(frame.options.cursor.style, x, y, width.max(cell_w), cell_h);
-                ctx.fill_rect(rect.x, rect.y, rect.width, rect.height);
-            } else if set_fill_color(ctx, themed_color(&segment.style.background, frame.options)) {
-                ctx.fill_rect(x, y, width, cell_h);
-            }
-            if segment.style.underline {
-                if segment.is_cursor {
-                    ctx.set_fill_style_str(&frame.options.theme.cursor_foreground);
-                } else {
-                    set_fill_color(ctx, themed_color(&segment.style.color, frame.options));
-                }
-                ctx.fill_rect(x, y + cell_h - 1.0, width, 1.0);
-            }
-            col += segment.cols;
-        }
-    }
-}
-
 fn cursor_cell_text(row: &TerminalRow, cursor_col: usize) -> Option<String> {
     let mut col = 0usize;
     for segment in &row.segments {
@@ -1523,6 +1653,103 @@ mod tests {
         assert_eq!(&vertices[0..8], &[-1.0, 0.0, 0.0, 0.5, 1.0, 0.5, 0.0, 1.0]);
     }
 
+    #[test]
+    fn push_solid_quad_uses_full_white_texture_uvs() {
+        let dom = CanvasDomState {
+            backing_width: 200,
+            backing_height: 100,
+            css_width: 100,
+            css_height: 50,
+            top_px: 0.0,
+        };
+        let mut vertices = Vec::new();
+        push_solid_quad(
+            &mut vertices,
+            SolidQuad {
+                x: 0.0,
+                y: 0.0,
+                width: 50.0,
+                height: 25.0,
+                color: [0.25, 0.5, 0.75, 1.0],
+            },
+            dom,
+        );
+
+        assert_eq!(vertices.len(), WEBGL_VERTEX_FLOATS * 6);
+        assert_eq!(
+            &vertices[0..8],
+            &[-1.0, 0.0, 0.0, 1.0, 0.25, 0.5, 0.75, 1.0]
+        );
+    }
+
+    #[test]
+    fn collect_backdrop_quads_skips_transparent_and_tracks_cursor_overlay() {
+        let mut options = default_options();
+        options.cursor.style = TerminalCursorStyle::Underline;
+        options.theme.cursor_background = "#ffffff".to_owned();
+        options.theme.cursor_foreground = "#000000".to_owned();
+        let rows = vec![TerminalRow {
+            segments: vec![
+                TerminalSegment {
+                    text: "ab".to_owned(),
+                    cols: 2,
+                    style: TerminalStyle {
+                        background: "transparent".to_owned(),
+                        underline: true,
+                        color: "#80ff00".to_owned(),
+                        ..TerminalStyle::default()
+                    },
+                    is_cursor: false,
+                },
+                TerminalSegment {
+                    text: "c".to_owned(),
+                    cols: 1,
+                    style: TerminalStyle::default(),
+                    is_cursor: true,
+                },
+            ],
+        }];
+        let frame = CanvasSurfaceFrame {
+            dom_state: CanvasDomState {
+                backing_width: 300,
+                backing_height: 90,
+                css_width: 100,
+                css_height: 30,
+                top_px: 0.0,
+            },
+            dpr: 1.0,
+            visible: &rows,
+            cell_size: (10.0, 18.0),
+            content_top_px: 4.0,
+            cursor_pos: Some((0, 2)),
+            options: &options,
+        };
+
+        let quads = collect_backdrop_quads(frame).expect("backdrop quads");
+
+        assert_eq!(quads.len(), 2);
+        assert_eq!(
+            quads[0],
+            SolidQuad {
+                x: 0.0,
+                y: 21.0,
+                width: 20.0,
+                height: 1.0,
+                color: [128.0 / 255.0, 1.0, 0.0, 1.0],
+            }
+        );
+        assert_eq!(
+            quads[1],
+            SolidQuad {
+                x: 20.0,
+                y: 19.0,
+                width: 10.0,
+                height: 3.0,
+                color: [1.0, 1.0, 1.0, 1.0],
+            }
+        );
+    }
+
     #[test]
     fn canvas_viewport_top_pins_live_mode_to_bottom() {
         let total_px = 1000.0 * DEFAULT_LINE_HEIGHT_PX;