feat: Add nesting of tags in a tree

- levels are joined by forward slashes in the keywords
- behaviour is opt-in for now
- long paths are shortened if there is not enough display room

Closes #687.

hooks/__tests__/use-shortened-text.test.tsx

@@ -0,0 +1,70 @@
+import { render, screen } from '@testing-library/react';
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { useShortenedText } from '../use-shortened-text';
+
+const CANDIDATES = ['Work/Clients/Acme/Sales', 'Work/../Acme/Sales', 'Work/.../Sales'];
+
+/**
+ * Reports `width` for the observed element and measures text at 10px per
+ * character, so a width of N*10 fits any candidate of N characters or fewer.
+ */
+function stubMeasurement(width: number) {
+  // Implementing the interface rather than passing an anonymous class keeps the
+  // members the hook never calls from reading as dead code.
+  class StubResizeObserver implements ResizeObserver {
+    constructor(private readonly callback: ResizeObserverCallback) {}
+
+    /** The hook observes once on mount; hand it `width` straight back. */
+    observe(target: Element) {
+      this.callback([{ target, contentRect: { width } } as unknown as ResizeObserverEntry], this);
+    }
+
+    unobserve() {}
+
+    disconnect() {}
+  }
+
+  vi.stubGlobal('ResizeObserver', StubResizeObserver);
+  vi.spyOn(HTMLCanvasElement.prototype, 'getContext').mockReturnValue({
+    font: '',
+    measureText: (text: string) => ({ width: text.length * 10 }),
+  } as unknown as CanvasRenderingContext2D);
+}
+
+function Probe({ candidates }: { candidates: string[] }) {
+  const [ref, text] = useShortenedText(candidates);
+  return <span ref={ref} data-testid="probe">{text}</span>;
+}
+
+function renderProbe(candidates: string[]): string {
+  render(<Probe candidates={candidates} />);
+  return screen.getByTestId('probe').textContent ?? '';
+}
+
+describe('useShortenedText', () => {
+  afterEach(() => {
+    vi.unstubAllGlobals();
+    vi.restoreAllMocks();
+  });
+
+  it('returns the longest candidate where the DOM cannot be measured', () => {
+    // No ResizeObserver: server rendering, and jsdom by default. Showing the
+    // whole path beats shortening it on a guess.
+    expect(renderProbe(CANDIDATES)).toBe('Work/Clients/Acme/Sales');
+  });
+
+  it('keeps the full path when the element is wide enough', () => {
+    stubMeasurement(230);
+    expect(renderProbe(CANDIDATES)).toBe('Work/Clients/Acme/Sales');
+  });
+
+  it('steps down only as far as the width requires', () => {
+    stubMeasurement(200);
+    expect(renderProbe(CANDIDATES)).toBe('Work/../Acme/Sales');
+  });
+
+  it('falls back to the shortest candidate when none of them fit', () => {
+    stubMeasurement(40);
+    expect(renderProbe(CANDIDATES)).toBe('Work/.../Sales');
+  });
+});

hooks/use-keyword-format.ts

@@ -0,0 +1,30 @@
+"use client";
+
+import { useMemo } from "react";
+import { useSettingsStore } from "@/stores/settings-store";
+import { formatKeyword, formatKeywordLabels, keywordRenderings } from "@/lib/keyword-format";
+
+/**
+ * Names tags for the screen, bound to the user's tag settings.
+ *
+ * Resolving the definitions and the nesting setting here rather than at every
+ * call site means no caller can forget the setting and render a nested name to
+ * someone who never asked for nesting. Subscribing to it also keeps names in
+ * step the moment it is toggled: reading it straight from the store inside the
+ * formatter would leave every list showing stale names until something else
+ * happened to re-render them.
+ */
+export function useKeywordFormat() {
+  const keywords = useSettingsStore((state) => state.emailKeywords);
+  const nested = useSettingsStore((state) => state.nestedTags);
+
+  return useMemo(
+    () => ({
+      /** The tag's display name. */
+      tagName: (id: string) => formatKeyword(id, keywords, nested),
+      /** Its progressively shorter forms, longest first, for `useShortenedText`. */
+      tagNameCandidates: (id: string) => keywordRenderings(formatKeywordLabels(id, keywords, nested)),
+    }),
+    [keywords, nested],
+  );
+}

hooks/use-shortened-text.ts

@@ -0,0 +1,76 @@
+"use client";
+
+import { useEffect, useMemo, useState } from "react";
+
+/**
+ * Measures text the way the browser will, using the font the element actually
+ * renders with. One canvas is reused for every measurement.
+ */
+let measureContext: CanvasRenderingContext2D | null | undefined;
+
+function measureText(text: string, font: string): number {
+  if (measureContext === undefined) {
+    measureContext = document.createElement("canvas").getContext("2d");
+  }
+  if (!measureContext) return 0;
+  measureContext.font = font;
+  return measureContext.measureText(text).width;
+}
+
+/**
+ * Picks the first of `candidates` that fits the element the returned ref is
+ * attached to, remeasuring whenever that element is resized.
+ *
+ * Candidates run longest first, so the result is the most complete one there is
+ * room for. A character budget cannot do this job: the columns this is used in
+ * are resized by the user and share their row with controls whose width depends
+ * on the locale, so any fixed number is either so generous that it never
+ * triggers or so tight that it shortens text that would have fit.
+ *
+ * Attach the ref to an element whose width does *not* depend on its own text -
+ * a flex child that is allowed to shrink, i.e. one with `truncate` or
+ * `min-w-0`. On anything else, picking a shorter candidate would change the
+ * width that picked it and the two would oscillate.
+ *
+ * Where measurement is unavailable - server rendering, and jsdom under test -
+ * this returns the first candidate, so the text is complete rather than
+ * arbitrarily shortened.
+ */
+export function useShortenedText(
+  candidates: string[],
+): [(node: HTMLElement | null) => void, string] {
+  const [element, setElement] = useState<HTMLElement | null>(null);
+  const [box, setBox] = useState<{ width: number; font: string } | null>(null);
+
+  useEffect(() => {
+    if (!element || typeof ResizeObserver === "undefined") return;
+
+    const observer = new ResizeObserver((entries) => {
+      const entry = entries[0];
+      if (!entry) return;
+      const style = window.getComputedStyle(element);
+      setBox({
+        width: entry.contentRect.width,
+        font: `${style.fontStyle} ${style.fontWeight} ${style.fontSize} ${style.fontFamily}`,
+      });
+    });
+    observer.observe(element);
+    return () => observer.disconnect();
+  }, [element]);
+
+  // Candidates are rebuilt on every render, so key the choice on their content.
+  // They must not contain a newline, which keeps this join unambiguous.
+  const key = candidates.join("\n");
+
+  return [
+    setElement,
+    useMemo(() => {
+      const options = key.split("\n");
+      if (!box || box.width === 0) return options[0];
+      return (
+        options.find((option) => measureText(option, box.font) <= box.width)
+        ?? options[options.length - 1]
+      );
+    }, [key, box]),
+  ];
+}