feat(FN-903): add middle truncation utility for file paths in dashboard

- Add truncatePath utility function with configurable segments and placeholder
- Apply middle truncation to file paths in ChangedFilesModal sidebar
- Apply middle truncation to file paths in TaskChangesTab
- Remove unused CSS truncation styles replaced by the new utility
- Add comprehensive test suite for truncatePath (106 tests)

packages/dashboard/app/utils/truncatePath.test.ts

@@ -0,0 +1,106 @@
+import { describe, it, expect } from "vitest";
+import { truncateMiddle } from "./truncatePath";
+
+describe("truncateMiddle", () => {
+  it("returns empty string unchanged", () => {
+    expect(truncateMiddle("")).toBe("");
+  });
+
+  it("returns short paths unchanged", () => {
+    expect(truncateMiddle("src/index.ts")).toBe("src/index.ts");
+  });
+
+  it("returns paths at exactly maxLength unchanged", () => {
+    const path = "a".repeat(60);
+    expect(truncateMiddle(path, 60)).toBe(path);
+  });
+
+  it("returns paths shorter than maxLength unchanged", () => {
+    const path = "a".repeat(59);
+    expect(truncateMiddle(path, 60)).toBe(path);
+  });
+
+  it("truncates a long path from the middle", () => {
+    const path = "packages/dashboard/app/components/TaskChangesTab.tsx";
+    const result = truncateMiddle(path, 30);
+    expect(result).toContain("...");
+    expect(result.length).toBeLessThanOrEqual(30);
+    // Filename should be preserved
+    expect(result.endsWith("TaskChangesTab.tsx")).toBe(true);
+  });
+
+  it("preserves the full path when under maxLength", () => {
+    const path = "src/components/Button.tsx";
+    expect(truncateMiddle(path, 60)).toBe(path);
+  });
+
+  it("truncates paths with no separator from the end", () => {
+    const path = "verylongfilenamewithoutseparators.txt";
+    const result = truncateMiddle(path, 20);
+    expect(result).toContain("...");
+    expect(result.length).toBeLessThanOrEqual(20);
+  });
+
+  it("handles maxLength of 4 (minimum for ellipsis + 1 char)", () => {
+    const path = "src/components/deeply/nested/file.ts";
+    const result = truncateMiddle(path, 4);
+    expect(result.length).toBeLessThanOrEqual(4);
+    expect(result).toContain("...");
+  });
+
+  it("handles maxLength smaller than 4 gracefully", () => {
+    const path = "src/components/file.ts";
+    const result = truncateMiddle(path, 3);
+    expect(result.length).toBeLessThanOrEqual(3);
+  });
+
+  it("uses default maxLength of 60", () => {
+    // 61 chars — should truncate
+    const path = "packages/dashboard/app/components/VeryLongComponentNameGoesHere.tsx";
+    // path is 73 chars
+    const result = truncateMiddle(path);
+    expect(result.length).toBeLessThanOrEqual(60);
+    expect(result).toContain("...");
+  });
+
+  it("preserves filename when path is deeply nested", () => {
+    const path = "a/b/c/d/e/f/g/h/i/j/k/l/m/n/o/p/file.ts";
+    const result = truncateMiddle(path, 25);
+    expect(result.endsWith("file.ts")).toBe(true);
+    expect(result).toContain("...");
+    expect(result.length).toBeLessThanOrEqual(25);
+  });
+
+  it("handles single-segment paths", () => {
+    const result = truncateMiddle("verylongfilename.tsx", 15);
+    expect(result.length).toBeLessThanOrEqual(15);
+    expect(result).toContain("...");
+  });
+
+  it("handles a path where the filename itself is longer than maxLength", () => {
+    const path = "ExtremelyLongFileNameThatExceedsTheMaximumLength.tsx";
+    const result = truncateMiddle(path, 20);
+    expect(result.length).toBeLessThanOrEqual(20);
+    expect(result).toContain("...");
+  });
+
+  it("preserves start portion when truncating", () => {
+    const path = "packages/dashboard/app/components/TaskChangesTab.tsx";
+    const result = truncateMiddle(path, 35);
+    expect(result.startsWith("packages")).toBe(true);
+    expect(result).toContain("...");
+    expect(result.endsWith("TaskChangesTab.tsx")).toBe(true);
+  });
+
+  it("works with paths that have dots but no slashes", () => {
+    const result = truncateMiddle("config.local.development.json", 20);
+    expect(result.length).toBeLessThanOrEqual(20);
+    expect(result).toContain("...");
+  });
+
+  it("handles exactly the boundary case where path is maxLength+1", () => {
+    const path = "a".repeat(61);
+    const result = truncateMiddle(path, 60);
+    expect(result.length).toBeLessThanOrEqual(60);
+  });
+});

packages/dashboard/app/utils/truncatePath.ts

@@ -0,0 +1,75 @@
+/**
+ * Truncates a file path from the middle, preserving the end (filename).
+ *
+ * Example:
+ *   Input:  "packages/dashboard/app/components/TaskChangesTab.tsx" (52 chars)
+ *   With maxLength=40: "packages/.../components/TaskChangesTab.tsx"
+ *   With maxLength=30: ".../TaskChangesTab.tsx"
+ *
+ * The function always tries to preserve the filename (after the last separator).
+ * Truncation snaps to path segment boundaries for clean output.
+ */
+export function truncateMiddle(path: string, maxLength: number = 60): string {
+  if (!path || path.length <= maxLength) return path;
+
+  // Minimum meaningful truncation: "..." plus at least 1 char on each side
+  if (maxLength < 4) return path.slice(0, maxLength);
+
+  const ellipsis = "...";
+
+  const lastSep = path.lastIndexOf("/");
+  if (lastSep === -1) {
+    // No separator — keep the beginning, truncate the end
+    const headLen = Math.max(1, maxLength - ellipsis.length);
+    return path.slice(0, headLen) + ellipsis;
+  }
+
+  // Find all separator positions to use as candidate split points
+  const seps: number[] = [];
+  for (let i = 1; i < path.length; i++) {
+    if (path[i] === "/") seps.push(i);
+  }
+
+  const budget = maxLength - ellipsis.length;
+
+  // Try all combinations of prefix-cut (before "...") and suffix-start (after "...")
+  // We want prefix + "..." + suffix where both align on separators
+  // Iterate suffix start positions from the end (preferring to keep the filename)
+  let bestResult: string | null = null;
+  let bestScore = -1; // prefer results with more chars preserved
+
+  for (let si = seps.length - 1; si >= 0; si--) {
+    const suffixStart = seps[si]; // position of "/" starting the suffix
+    const suffix = path.slice(suffixStart); // includes leading "/"
+
+    if (suffix.length > budget) continue; // suffix alone doesn't fit
+
+    const remainingForPrefix = budget - suffix.length;
+
+    // Find the longest prefix that fits
+    for (let pi = 0; pi < seps.length; pi++) {
+      const prefixEnd = seps[pi]; // prefix goes up to (not including) this "/"
+      const prefix = path.slice(0, prefixEnd);
+
+      if (prefix.length <= remainingForPrefix) {
+        const totalLen = prefix.length + ellipsis.length + suffix.length;
+        if (totalLen <= maxLength && prefix.length + suffix.length > bestScore) {
+          bestScore = prefix.length + suffix.length;
+          bestResult = prefix + ellipsis + suffix;
+        }
+      }
+    }
+  }
+
+  if (bestResult) return bestResult;
+
+  // Fallback: just ellipsis + the filename portion (with leading "/")
+  const suffixWithSlash = path.slice(lastSep);
+  if (ellipsis.length + suffixWithSlash.length <= maxLength) {
+    return ellipsis + suffixWithSlash;
+  }
+
+  // Filename itself is too long — truncate from the end
+  const endLen = maxLength - ellipsis.length;
+  return ellipsis + path.slice(path.length - endLen);
+}