fix: do not tint custom SVGs that have an opaque background

A grayscale SVG with an opaque full-canvas background (for example an
exported logo with a white background rect and a black glyph) passed the
monochrome check and was drawn through a CSS mask. Masks key off the alpha
channel, so the opaque background filled the whole area with the tint color
and the icon collapsed into a solid block.

Detect a full-canvas opaque background rect and exclude such SVGs from
tinting, rendering them with their own colors instead. Transparent
single-color and multi-shade glyphs remain tintable. Add tests for the
background cases.

client/src/utils/__tests__/svg.test.ts

@@ -81,6 +81,44 @@ describe('isMonochromeSvg', () => {
       expect(isMonochromeSvg(svg)).toBe(false);
     });
   });
+
+  describe('opaque background (not tintable as a mask)', () => {
+    it('rejects a white background rect with a black glyph', () => {
+      const svg =
+        '<svg viewBox="0 0 24 24"><rect width="24" height="24" fill="#ffffff" /><path fill="#000" d="M4 4h16v16H4z" /></svg>';
+      expect(isMonochromeSvg(svg)).toBe(false);
+    });
+
+    it('rejects a black background rect with a white glyph', () => {
+      const svg =
+        '<svg viewBox="0 0 48 48"><rect width="48" height="48" fill="black" /><path fill="white" d="M0 0h10v10H0z" /></svg>';
+      expect(isMonochromeSvg(svg)).toBe(false);
+    });
+
+    it('rejects a full-size percentage background rect', () => {
+      const svg =
+        '<svg viewBox="0 0 24 24"><rect x="0" y="0" width="100%" height="100%" fill="#eee" /><path fill="#222" /></svg>';
+      expect(isMonochromeSvg(svg)).toBe(false);
+    });
+
+    it('rejects a default-fill (opaque black) background rect', () => {
+      const svg =
+        '<svg viewBox="0 0 16 16"><rect width="16" height="16" /><path fill="#fff" /></svg>';
+      expect(isMonochromeSvg(svg)).toBe(false);
+    });
+
+    it('still tints a glyph drawn with small rects on a transparent background', () => {
+      const svg =
+        '<svg viewBox="0 0 24 24"><rect x="4" y="4" width="6" height="6" fill="#333" /><rect x="14" y="14" width="6" height="6" fill="#333" /></svg>';
+      expect(isMonochromeSvg(svg)).toBe(true);
+    });
+
+    it('ignores a transparent background rect', () => {
+      const svg =
+        '<svg viewBox="0 0 24 24"><rect width="24" height="24" fill="none" /><path fill="#333" /></svg>';
+      expect(isMonochromeSvg(svg)).toBe(true);
+    });
+  });
 });
 
 describe('sanitizeSvg', () => {

client/src/utils/svg.ts

@@ -103,12 +103,75 @@ function extractColors(svg: string): string[] {
   return colors;
 }
 
+const VIEWBOX_REGEX = /viewBox\s*=\s*["']\s*[-\d.]+\s+[-\d.]+\s+([\d.]+)\s+([\d.]+)/i;
+const RECT_REGEX = /<rect\b[^>]*>/gi;
+
+function getAttr(tag: string, name: string): string | null {
+  const match = tag.match(new RegExp(`\\b${name}\\s*=\\s*["']([^"']*)["']`, 'i'));
+  return match ? match[1].trim() : null;
+}
+
+function coversCanvas(value: string | null, canvas: number | null): boolean {
+  if (value == null) {
+    return false;
+  }
+  if (value.trim() === '100%') {
+    return true;
+  }
+  const size = parseFloat(value);
+  if (Number.isNaN(size)) {
+    return false;
+  }
+  return canvas != null && size >= canvas * 0.98;
+}
+
 /**
- * Returns true when an SVG only contains grayscale colors (or relies on the
- * default black fill / `currentColor`), meaning it can be safely tinted to match
- * the theme. Multi-color logos return false so their colors are preserved.
+ * Detects a full-canvas opaque background (a `<rect>` at the origin spanning the
+ * viewBox). Such SVGs cannot be tinted via a CSS mask, since the opaque
+ * background fills the whole area with the tint color instead of the glyph.
+ */
+function hasOpaqueBackground(svg: string): boolean {
+  const viewBox = svg.match(VIEWBOX_REGEX);
+  const width = viewBox ? parseFloat(viewBox[1]) : null;
+  const height = viewBox ? parseFloat(viewBox[2]) : null;
+
+  const rects = svg.match(RECT_REGEX) ?? [];
+  for (const rect of rects) {
+    const fill = getAttr(rect, 'fill');
+    if (fill === 'none' || fill === 'transparent') {
+      continue;
+    }
+    const fillOpacity = getAttr(rect, 'fill-opacity');
+    const opacity = getAttr(rect, 'opacity');
+    if (fillOpacity != null && parseFloat(fillOpacity) === 0) {
+      continue;
+    }
+    if (opacity != null && parseFloat(opacity) === 0) {
+      continue;
+    }
+    if (parseFloat(getAttr(rect, 'x') ?? '0') > 0 || parseFloat(getAttr(rect, 'y') ?? '0') > 0) {
+      continue;
+    }
+    if (
+      coversCanvas(getAttr(rect, 'width'), width) &&
+      coversCanvas(getAttr(rect, 'height'), height)
+    ) {
+      return true;
+    }
+  }
+  return false;
+}
+
+/**
+ * Returns true when an SVG can be safely tinted to match the theme: it only
+ * contains grayscale colors (or relies on the default black fill /
+ * `currentColor`) and has no opaque background. Multi-color logos and icons with
+ * an opaque background return false so they are rendered with their own colors.
  */
 export function isMonochromeSvg(svg: string): boolean {
+  if (hasOpaqueBackground(svg)) {
+    return false;
+  }
   const colors = extractColors(svg);
   if (colors.length === 0) {
     return true;