fix(renderer): paint reverse-video cell background with default colors

lib/renderer.ts

@@ -782,8 +782,18 @@ export class CanvasRenderer {
     // we cannot infer it from the RGB triple because (0,0,0) is a valid
     // explicit color (programs emit it for "true black" backgrounds, e.g.
     // letterboxed image renderings).
-    const useThemeBg = cell.flags & CellFlags.INVERSE ? cell.fgIsDefault : cell.bgIsDefault;
-    if (!useThemeBg) {
+    if (cell.flags & CellFlags.INVERSE) {
+      // Inverse: the cell's background is its foreground color. A *default*
+      // foreground resolves to theme.foreground — which is NOT the line-level
+      // theme.background fill — so it must be painted explicitly. The old code
+      // treated `fgIsDefault` as "use theme background" and skipped the fill,
+      // so reverse-video cells (and program-drawn block cursors, e.g. Claude
+      // Code's) rendered the dark background instead of a solid block.
+      this.ctx.fillStyle = cell.fgIsDefault
+        ? this.theme.foreground
+        : this.rgbToCSS(bg_r, bg_g, bg_b);
+      this.ctx.fillRect(cellX, cellY, cellWidth, this.metrics.height);
+    } else if (!cell.bgIsDefault) {
       this.ctx.fillStyle = this.rgbToCSS(bg_r, bg_g, bg_b);
       this.ctx.fillRect(cellX, cellY, cellWidth, this.metrics.height);
     }
@@ -844,7 +854,13 @@ export class CanvasRenderer {
       // when the cell has the default fg (tag NONE), not when its explicit
       // RGB happens to be (0,0,0).
       const useThemeFg = cell.flags & CellFlags.INVERSE ? cell.bgIsDefault : cell.fgIsDefault;
-      this.ctx.fillStyle = useThemeFg ? this.theme.foreground : this.rgbToCSS(fg_r, fg_g, fg_b);
+      // For an inverse cell the text takes the cell's *background* color; a
+      // default background resolves to theme.background (the swapped theme
+      // color), not theme.foreground — so the glyph stays legible against the
+      // inverted (theme.foreground) block painted in renderCellBackground.
+      const themeColor =
+        cell.flags & CellFlags.INVERSE ? this.theme.background : this.theme.foreground;
+      this.ctx.fillStyle = useThemeFg ? themeColor : this.rgbToCSS(fg_r, fg_g, fg_b);
     }
 
     // Apply faint effect

tests/e2e/01-rendering.spec.ts

@@ -28,6 +28,36 @@ test.describe('Rendering', () => {
     expect(await hasRenderedContent(page)).toBe(true);
   });
 
+  // Regression: a reverse-video cell with DEFAULT colors must paint its
+  // background with theme.foreground (a solid light block). This is how
+  // reverse-video TUIs — and program-drawn block cursors like Claude Code's —
+  // render their cursor. A prior bug treated `fgIsDefault` as "use theme
+  // background" and skipped the fill, leaving the block dark/empty.
+  test('reverse-video cell with default colors paints a solid (light) background', async ({
+    page,
+  }) => {
+    // 10 reverse-video spaces at row 0: no glyphs, so the sampled pixels are
+    // purely the inverted cell background.
+    await termWrite(page, '\x1b[7m          \x1b[0m');
+    const avgBrightness = await page.evaluate(() => {
+      const canvas = document.querySelector('#terminal-container canvas') as HTMLCanvasElement;
+      const ctx = canvas.getContext('2d')!;
+      // Sample a band inside the first cells, well within the reverse run and
+      // before the trailing cursor.
+      const { data } = ctx.getImageData(2, 2, 40, 10);
+      let sum = 0;
+      let n = 0;
+      for (let i = 0; i < data.length; i += 4) {
+        sum += (data[i] + data[i + 1] + data[i + 2]) / 3;
+        n++;
+      }
+      return sum / n;
+    });
+    // Default theme: background ≈ 30, foreground ≈ 212. A painted inverse
+    // background reads bright; the pre-fix bug left it at the dark background.
+    expect(avgBrightness).toBeGreaterThan(120);
+  });
+
   test('plain text appears in buffer', async ({ page }) => {
     await termWrite(page, 'Hello World');
     const line = await getLine(page, 0);