fix: rebalance Pierre word highlights in split view

CHANGELOG.md

@@ -14,6 +14,8 @@ All notable user-visible changes to Hunk are documented in this file.
 
 ### Fixed
 
+- Balanced Pierre word-level highlights so split-view inline changes stay visible without overpowering the surrounding diff row.
+
 ## [0.9.3] - 2026-04-13
 
 ### Fixed

src/ui/components/ui-components.test.tsx

@@ -5,6 +5,7 @@ import { act, createRef, useEffect, useState, type ReactNode } from "react";
 import type { AppBootstrap, DiffFile } from "../../core/types";
 import { createTestGitAppBootstrap } from "../../../test/helpers/app-bootstrap";
 import { createTestDiffFile as buildTestDiffFile, lines } from "../../../test/helpers/diff-helpers";
+import { hexColorDistance } from "../lib/color";
 import { resolveTheme } from "../themes";
 import { measureDiffSectionGeometry } from "../lib/diffSectionGeometry";
 import { buildFileSectionLayouts, buildInStreamFileHeaderHeights } from "../lib/fileSectionLayout";
@@ -351,6 +352,44 @@ function frameHasHighlightedMarker(
   });
 }
 
+/** Convert captured RGBA output back into a #rrggbb color string for contrast assertions. */
+function capturedColorToHex(color: { buffer?: ArrayLike<number> } | undefined) {
+  const buffer = color?.buffer;
+  if (!buffer || buffer[0] == null || buffer[1] == null || buffer[2] == null) {
+    return null;
+  }
+
+  const componentToHex = (value: number) =>
+    Math.max(0, Math.min(255, Math.round(value * 255)))
+      .toString(16)
+      .padStart(2, "0");
+
+  return `#${componentToHex(buffer[0])}${componentToHex(buffer[1])}${componentToHex(buffer[2])}`;
+}
+
+/** Measure the rendered background contrast between one word-diff span and its surrounding line. */
+function renderedWordDiffBackgroundDistance(
+  frame: { lines: Array<{ spans: Array<{ text: string; bg?: { buffer?: ArrayLike<number> } }> }> },
+  marker: string,
+) {
+  for (const line of frame.lines) {
+    const spanIndex = line.spans.findIndex((span) => span.text.includes(marker));
+    if (spanIndex <= 0) {
+      continue;
+    }
+
+    const wordBg = capturedColorToHex(line.spans[spanIndex]?.bg);
+    const surroundingBg = capturedColorToHex(line.spans[spanIndex - 1]?.bg);
+    if (!wordBg || !surroundingBg) {
+      continue;
+    }
+
+    return hexColorDistance(wordBg, surroundingBg);
+  }
+
+  return null;
+}
+
 describe("UI components", () => {
   test("SidebarPane renders grouped file rows with indented filenames and right-aligned stats", async () => {
     const theme = resolveTheme("midnight", null);
@@ -1901,6 +1940,61 @@ describe("UI components", () => {
     expect(binaryFileFrame).toContain("Binary file skipped");
   });
 
+  test("PierreDiffView renders word-diff spans with a visibly different background in split view", async () => {
+    const file = createTestDiffFile(
+      "word-diff",
+      "word-diff.ts",
+      "export const answer = 41;\nexport const stable = true;\n",
+      "export const answer = 42;\nexport const stable = true;\n",
+    );
+    const theme = resolveTheme("graphite", null);
+    const setup = await testRender(
+      <PierreDiffView
+        file={file}
+        layout="split"
+        theme={theme}
+        width={120}
+        selectedHunkIndex={0}
+        scrollable={false}
+      />,
+      { width: 124, height: 10 },
+    );
+
+    try {
+      let removedBackgroundDistance: number | null = null;
+      let addedBackgroundDistance: number | null = null;
+
+      for (let iteration = 0; iteration < 200; iteration += 1) {
+        await act(async () => {
+          await setup.renderOnce();
+          await Bun.sleep(0);
+          await setup.renderOnce();
+          await Bun.sleep(0);
+        });
+
+        const frame = setup.captureSpans();
+        removedBackgroundDistance = renderedWordDiffBackgroundDistance(frame, "41");
+        addedBackgroundDistance = renderedWordDiffBackgroundDistance(frame, "42");
+
+        if (
+          removedBackgroundDistance !== null &&
+          addedBackgroundDistance !== null &&
+          removedBackgroundDistance > 0 &&
+          addedBackgroundDistance > 0
+        ) {
+          break;
+        }
+      }
+
+      expect(removedBackgroundDistance).toBeGreaterThanOrEqual(28);
+      expect(addedBackgroundDistance).toBeGreaterThanOrEqual(28);
+    } finally {
+      await act(async () => {
+        setup.renderer.destroy();
+      });
+    }
+  });
+
   test("PierreDiffView reuses highlighted rows after unmounting and remounting a file section", async () => {
     const file = createTestDiffFile(
       "cache",

src/ui/diff/pierre.test.ts

@@ -115,10 +115,19 @@ describe("Pierre diff rows", () => {
       throw new Error("Expected a split-line change row");
     }
 
-    expect(changedRow.left.spans.some((span) => span.text.includes("41"))).toBe(true);
-    expect(changedRow.right.spans.some((span) => span.text.includes("42"))).toBe(true);
-    expect(changedRow.left.spans.some((span) => span.bg === theme.removedContentBg)).toBe(true);
-    expect(changedRow.right.spans.some((span) => span.bg === theme.addedContentBg)).toBe(true);
+    const removedWordSpan = changedRow.left.spans.find((span) => span.text.includes("41"));
+    const addedWordSpan = changedRow.right.spans.find((span) => span.text.includes("42"));
+
+    expect(removedWordSpan).toBeDefined();
+    expect(addedWordSpan).toBeDefined();
+    expect(removedWordSpan?.bg).toBeDefined();
+    expect(addedWordSpan?.bg).toBeDefined();
+    expect(changedRow.left.spans.some((span) => span.text.includes("export") && span.bg)).toBe(
+      false,
+    );
+    expect(changedRow.right.spans.some((span) => span.text.includes("export") && span.bg)).toBe(
+      false,
+    );
     expect(
       changedRow.right.spans.some(
         (span) => span.text.includes("export") && typeof span.fg === "string",

src/ui/diff/pierre.ts

@@ -7,6 +7,7 @@ import {
 } from "@pierre/diffs";
 import { formatHunkHeader } from "../../core/hunkHeader";
 import type { DiffFile } from "../../core/types";
+import { blendHex, hexColorDistance } from "../lib/color";
 import type { AppTheme } from "../themes";
 import { expandDiffTabs } from "./codeColumns";
 
@@ -165,6 +166,53 @@ const normalizedColorCache = new Map<string, Map<string, string>>();
 // into terminal spans. The same highlighted line objects are reused when files remount or when
 // we build both split and stack rows, so memoize flattened spans by line node + theme/background.
 const flattenedHighlightedLineCache = new WeakMap<HastNode, Map<string, RenderSpan[]>>();
+const MIN_WORD_DIFF_BG_DISTANCE = 28;
+const WORD_DIFF_BLEND_STEP = 0.005;
+const WORD_DIFF_MAX_BLEND = 0.2;
+const wordDiffBackgroundCache = new Map<string, Record<SplitLineCell["kind"], string>>();
+
+/** Blend toward the semantic sign color just enough to hit the minimum visible contrast. */
+function strengthenWordDiffBg(lineBg: string, signColor: string) {
+  let strongestCandidate = lineBg;
+  const maxSteps = Math.floor(WORD_DIFF_MAX_BLEND / WORD_DIFF_BLEND_STEP);
+
+  for (let step = 1; step <= maxSteps; step += 1) {
+    const blendRatio = step * WORD_DIFF_BLEND_STEP;
+    const candidate = blendHex(signColor, lineBg, blendRatio);
+    strongestCandidate = candidate;
+
+    if (hexColorDistance(candidate, lineBg) >= MIN_WORD_DIFF_BG_DISTANCE) {
+      return candidate;
+    }
+  }
+
+  return strongestCandidate;
+}
+
+/** Resolve the inline word-diff background, strengthening theme colors that are too subtle to see. */
+function wordDiffHighlightBg(kind: SplitLineCell["kind"], theme: AppTheme) {
+  let cached = wordDiffBackgroundCache.get(theme.id);
+  if (!cached) {
+    const addition =
+      hexColorDistance(theme.addedContentBg, theme.addedBg) >= MIN_WORD_DIFF_BG_DISTANCE
+        ? theme.addedContentBg
+        : strengthenWordDiffBg(theme.addedBg, theme.addedSignColor);
+    const deletion =
+      hexColorDistance(theme.removedContentBg, theme.removedBg) >= MIN_WORD_DIFF_BG_DISTANCE
+        ? theme.removedContentBg
+        : strengthenWordDiffBg(theme.removedBg, theme.removedSignColor);
+
+    cached = {
+      addition,
+      context: theme.contextContentBg,
+      deletion,
+      empty: theme.panelAlt,
+    };
+    wordDiffBackgroundCache.set(theme.id, cached);
+  }
+
+  return cached[kind];
+}
 
 /** Remap Pierre token hues that collide with diff add/remove semantics into theme-safe syntax colors. */
 function normalizeHighlightedColor(color: string | undefined, theme: AppTheme) {
@@ -301,15 +349,7 @@ function makeSplitCell(
     const fallbackText = cleanDiffLine(rawLine);
     spans = fallbackText.length > 0 ? [{ text: fallbackText }] : [];
   } else {
-    spans = flattenHighlightedLine(
-      highlightedLine,
-      theme,
-      kind === "addition"
-        ? theme.addedContentBg
-        : kind === "deletion"
-          ? theme.removedContentBg
-          : theme.contextContentBg,
-    );
+    spans = flattenHighlightedLine(highlightedLine, theme, wordDiffHighlightBg(kind, theme));
 
     if (spans.length === 0) {
       const fallbackText = cleanDiffLine(rawLine);
@@ -341,15 +381,7 @@ function makeStackCell(
     const fallbackText = cleanDiffLine(rawLine);
     spans = fallbackText.length > 0 ? [{ text: fallbackText }] : [];
   } else {
-    spans = flattenHighlightedLine(
-      highlightedLine,
-      theme,
-      kind === "addition"
-        ? theme.addedContentBg
-        : kind === "deletion"
-          ? theme.removedContentBg
-          : theme.contextContentBg,
-    );
+    spans = flattenHighlightedLine(highlightedLine, theme, wordDiffHighlightBg(kind, theme));
 
     if (spans.length === 0) {
       const fallbackText = cleanDiffLine(rawLine);

src/ui/diff/renderRows.tsx

@@ -7,6 +7,7 @@ import {
   resolveStackCellGeometry,
 } from "./codeColumns";
 import type { DiffRow, RenderSpan, SplitLineCell, StackLineCell } from "./pierre";
+import { blendHex } from "../lib/color";
 
 /** Clamp a label to one terminal row with an ellipsis. */
 export function fitText(text: string, width: number) {
@@ -79,23 +80,6 @@ function sliceSpansWindow(spans: RenderSpan[], offset: number, width: number) {
   };
 }
 
-/** Parse a hex color string into RGB components. */
-function hexToRgb(hex: string) {
-  const n = parseInt(hex.slice(1), 16);
-  return { r: (n >> 16) & 0xff, g: (n >> 8) & 0xff, b: n & 0xff };
-}
-
-/** Blend a color toward a background at a given ratio (0 = bg, 1 = fg). */
-function blendHex(fg: string, bg: string, ratio: number) {
-  const f = hexToRgb(fg);
-  const b = hexToRgb(bg);
-  const mix = (a: number, z: number) => Math.round(z + (a - z) * ratio);
-  const r = mix(f.r, b.r);
-  const g = mix(f.g, b.g);
-  const bl = mix(f.b, b.b);
-  return `#${((r << 16) | (g << 8) | bl).toString(16).padStart(6, "0")}`;
-}
-
 const INACTIVE_RAIL_BLEND = 0.35;
 
 /** Dim a rail color for inactive hunks by blending toward the panel background. */

src/ui/lib/color.ts

@@ -0,0 +1,40 @@
+/** One parsed RGB triplet from a #rrggbb hex color. */
+interface RgbColor {
+  r: number;
+  g: number;
+  b: number;
+}
+
+/** Parse a #rrggbb color into RGB components. Falls back to black for invalid input. */
+function hexToRgb(hex: string): RgbColor {
+  const normalized = /^#?[0-9a-f]{6}$/i.test(hex) ? hex.replace(/^#/, "") : "000000";
+  const value = parseInt(normalized, 16);
+  return {
+    r: (value >> 16) & 0xff,
+    g: (value >> 8) & 0xff,
+    b: value & 0xff,
+  };
+}
+
+/** Blend one foreground color toward a background color at a fixed ratio. */
+export function blendHex(fg: string, bg: string, ratio: number) {
+  const foreground = hexToRgb(fg);
+  const background = hexToRgb(bg);
+  const mix = (front: number, back: number) =>
+    Math.max(0, Math.min(255, Math.round(back + (front - back) * ratio)));
+
+  return `#${(
+    (mix(foreground.r, background.r) << 16) |
+    (mix(foreground.g, background.g) << 8) |
+    mix(foreground.b, background.b)
+  )
+    .toString(16)
+    .padStart(6, "0")}`;
+}
+
+/** Measure how visually separated two #rrggbb colors are using channel deltas. */
+export function hexColorDistance(left: string, right: string) {
+  const a = hexToRgb(left);
+  const b = hexToRgb(right);
+  return Math.abs(a.r - b.r) + Math.abs(a.g - b.g) + Math.abs(a.b - b.b);
+}