fix(runtime): backgroundColor="" on a Text opts out of the inherited Box bg (Ink parity) (#83)

An explicit backgroundColor="" on a descendant Text now renders bare glyphs instead of
bleeding the inherited Box background. Mirrors Ink Text.tsx:103-106 (effectiveBg =
backgroundColor ?? inheritedBg; colorize only when truthy): undefined inherits, ""
opts out. The trailing padding still uses the inherited Box bg (Box fill), so glyphs
carry their effective bg while the Box fills the rest -- byte-identical to Ink.

Corrects the background-color.test.tsx mixed snapshot, which had encoded the buggy
green-bleed output. Adds the opt-out test.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/runtime-tests/integration/components/background-color.test.tsx

@@ -146,6 +146,24 @@ test("Multiple Text elements inherit same background", async ({ expect }) => {
   expect(lastFrame()).toMatchInlineSnapshot(`"Hello World"`);
 });
 
+// Ink parity (Text.tsx:103-106): a child <Text>'s effective background is
+// `backgroundColor ?? inheritedBackgroundColor`, and the bg wrap is applied only
+// when that value is truthy. An explicit `backgroundColor=""` is NOT undefined,
+// so it does NOT inherit — it resolves to `""` (falsy) and OPTS OUT of the
+// inherited Box background, rendering the glyphs with no bg.
+test("Text backgroundColor='' opts out of inherited Box background", async ({ expect }) => {
+  const { lastFrame } = await render(
+    defineComponent(() => () => (
+      <Box backgroundColor="green" alignSelf="flex-start">
+        <Text backgroundColor="">No BG</Text>
+      </Box>
+    )),
+    { columns: 100 },
+  );
+  // Bare text, no green (42) bleed and no bg reset (49) at all.
+  expect(lastFrame()).toBe("No BG");
+});
+
 test("Mixed text with and without background inheritance", async ({ expect }) => {
   const { lastFrame } = await render(
     defineComponent(() => () => (
@@ -157,7 +175,9 @@ test("Mixed text with and without background inheritance", async ({ expect }) =>
     )),
     { columns: 100 },
   );
-  expect(lastFrame()).toMatchInlineSnapshot(`"Inherited No BG Red BG"`);
+  // Matches Ink background.tsx:106-116: bgGreen('Inherited ') + 'No BG ' + bgRed('Red BG').
+  // The "" Text opts out, so green is reset (49) before "No BG " and it renders bare.
+  expect(lastFrame()).toMatchInlineSnapshot(`"Inherited No BG Red BG"`);
 });
 
 test("Complex nested structure with background inheritance", async ({ expect }) => {

packages/runtime/src/paint/paint.ts

@@ -553,18 +553,37 @@ function paintNode(
     }
     case "text": {
       const layout = node.yoga.getComputedLayout();
-      const bgProps: TextProps = inheritedBg ? { backgroundColor: inheritedBg } : {};
+      // Ink Text.tsx:103-106: a Text's effective background is its OWN
+      // backgroundColor if defined (`??`, so an explicit "" is honored), else the
+      // inherited Box background; the bg is applied only when truthy. Passing the
+      // effective value as the squash base means an explicit "" opts OUT (renders
+      // bare glyphs) while `undefined` inherits — matching Ink's `??` semantics.
+      // (renderTextWithInlineStyles still lets the node's own props override this
+      // base, so the result is identical, but stating the effective value here
+      // keeps the intent explicit.)
+      const effectiveBg = (node.props["backgroundColor"] as string | undefined) ?? inheritedBg;
+      const bgProps: TextProps = effectiveBg ? { backgroundColor: effectiveBg } : {};
       const text = renderTextWithInlineStyles(node, bgProps);
       // Skip writing empty text — avoids applying line transformers to empty
       // content, which matches Ink's behavior of not writing empty text nodes.
       if (text === "") return;
       const cellWidth = Math.max(1, Math.floor(layout.width));
       const wrapped = wrapText(text, cellWidth, node.props.wrap ?? "wrap");
+      // Pad each line to the cell width with the INHERITED Box background only —
+      // this fills the space behind the text with the Box's bg (the Box also fills
+      // it via fillBackground), and is the reason a Box bg pads to full width while
+      // a text-only bg does not. The padding uses `inheritedBg`, NOT the effective
+      // bg: a Text that overrides or opts out (backgroundColor / "") only recolors
+      // its OWN glyphs, never the surrounding Box fill. The already-rendered glyphs
+      // in `wrapped[i]` keep their effective bg, so a `backgroundColor=""` Text
+      // stays bare even though we pad the trailing cells with the inherited bg.
       if (inheritedBg) {
+        const padProps: TextProps = { backgroundColor: inheritedBg };
         for (let i = 0; i < wrapped.length; i++) {
           const pad = cellWidth - stringWidth(wrapped[i]!);
-          const padStr = pad > 0 ? " ".repeat(pad) : "";
-          wrapped[i] = applyChalk(wrapped[i]! + padStr, bgProps);
+          if (pad > 0) {
+            wrapped[i] = wrapped[i]! + applyChalk(" ".repeat(pad), padProps);
+          }
         }
       }
       output.write(x0 + layout.left, y0 + layout.top, wrapped, transformers);