refactor(chips): share the chip-overflow measure core between the two lanes (S3)

ActiveFilterChips (feed applied-filter row) and ChannelTagsControl both
hand-rolled the same single-line chip-fit measurer: a hidden mirror + a
layout-effect + ResizeObserver reading chip right edges, the real rem gap,
the pill width and the lane width. The policy half was already extracted
(decideChipLayout); the DOM-measurement half was still duplicated.

Promote that half to useChipOverflowFit(rowRef, chipsRef, pillRef, deps,
decide, initial): it owns the effect + observer + initial measure and reads
the common geometry, then hands it to each caller's own policy. The feed
row layers its leading label + trailing "Clear all" reservation on top and
keeps decideChipLayout; the tag cell's chips-only policy is extracted as
fitChipsWithPill (pure, unit-tested) alongside it. Add a jsdom test for the
hook (stubbed offsets, re-measure on deps). Behaviour unchanged — verified
in the browser: feed all/count modes + popover, tag "+N" fit + hover reveal.

frontend/src/lib/chipOverflow.test.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from "vitest";
-import { decideChipLayout, type ChipFitInput } from "./chipOverflow";
+import { decideChipLayout, fitChipsWithPill, type ChipFitInput } from "./chipOverflow";
 
 // Build measurer-style right edges for `n` chips of a fixed width, starting after a label, with a gap.
 function rights(labelRight: number, widths: number[], gap: number): number[] {
@@ -91,3 +91,29 @@ describe("decideChipLayout", () => {
     expect(decideChipLayout(input)).toEqual({ mode: "count", visible: 0 });
   });
 });
+
+describe("fitChipsWithPill", () => {
+  // Three chips of width 50, gap 6, starting at 0 -> right edges 50, 106, 162.
+  const chipRights = rights(0, [50, 50, 50], 6);
+
+  it("shows all chips (no pill) when the last chip's right edge fits — the whole-set fast path", () => {
+    expect(fitChipsWithPill({ chipRights, pillW: 40, gap: 6, avail: 200 })).toBe(3);
+  });
+
+  it("returns 0 for an empty chip set", () => {
+    expect(fitChipsWithPill({ chipRights: [], pillW: 40, gap: 6, avail: 200 })).toBe(0);
+  });
+
+  it("reserves the pill width for all but the last chip when overflowing", () => {
+    // avail 120: whole set (162) doesn't fit. chip0 right 50 <= 120-(40+6)=74 ok; chip1 right 106 > 74
+    // -> dropped at i=1 -> visible 1.
+    expect(fitChipsWithPill({ chipRights, pillW: 40, gap: 6, avail: 120 })).toBe(1);
+  });
+
+  it("keeps the whole set via the fast path even when a reserve-for-all loop would drop chips", () => {
+    // Right edges 56, 112, 118; avail 120: the whole set fits (118 <= 120). A naive loop that reserves
+    // pill+gap (46) for every non-last chip would drop at chip1 (112 > 120-46=74) and show "+2" beside
+    // empty space — the fast path returns all 3 instead.
+    expect(fitChipsWithPill({ chipRights: [56, 112, 118], pillW: 40, gap: 6, avail: 120 })).toBe(3);
+  });
+});

frontend/src/lib/chipOverflow.ts

@@ -1,11 +1,13 @@
-// Pure fit decision for a single-line chip lane that must share its width with fixed trailing
-// controls (the feed's applied-filter row + Sort/View). Lifted out of ActiveFilterChips so the
-// all/some/count math is unit-testable without a layout engine (jsdom reports 0-width boxes), the
-// same split as lib/dateFilter and lib/columnSort. The caller measures real widths off a hidden
-// mirror (including the live rem gap, so nothing is frozen in px) and hands them here.
-//
-// A sibling one-line-fit measurer lives in ChannelTags (ChannelTagsControl); if a third appears,
-// promote the DOM-measurement half to a shared hook and keep this as its policy core.
+// Pure fit decisions for a single-line chip lane that collapses its overflow into a "+N" pill.
+// Lifted out of the components so the math is unit-testable without a layout engine (jsdom reports
+// 0-width boxes), the same split as lib/dateFilter and lib/columnSort. The shared DOM-measurement
+// half — the layout-effect + ResizeObserver that reads real rem boxes off a hidden mirror — lives in
+// lib/useChipOverflowFit; it hands the measured geometry to one of these policies:
+//   - decideChipLayout: the feed's applied-filter row, which also shares its width with fixed trailing
+//     controls (Sort/View), reserves a leading label + a trailing "Clear all", and collapses to a
+//     single count pill on a narrow lane.
+//   - fitChipsWithPill: the chips-only channel tag cell (ChannelTagsControl) — just "how many fit
+//     before the +N pill".
 
 export interface ChipFitInput {
   /** Number of chips. */
@@ -51,3 +53,37 @@ export function decideChipLayout(p: ChipFitInput): ChipFit {
   if (k === 0 && labelRight + gap + plusW > avail) return { mode: "count", visible: 0 };
   return { mode: "some", visible: k };
 }
+
+// The chips-only policy (ChannelTagsControl): how many chips fit on the line, reserving the "+N" pill's
+// width whenever any overflow. Returns the visible count.
+export interface ChipsWithPillInput {
+  /** Right edge (offsetLeft + offsetWidth) of each chip, index-aligned. */
+  chipRights: number[];
+  /** Width of the "+N" pill. */
+  pillW: number;
+  /** Available lane width. */
+  avail: number;
+  /** The real (rem-scaled) inter-item gap. */
+  gap: number;
+}
+
+export function fitChipsWithPill(p: ChipsWithPillInput): number {
+  const { chipRights, pillW, avail, gap } = p;
+  const n = chipRights.length;
+  if (n === 0) return 0;
+  // Whole-set fast path: if the LAST chip's right edge already fits, every chip fits and NO pill is
+  // needed. The reservation loop below reserves the pill for every chip except the last and breaks at
+  // the first overflow, so on its own it would collapse a short trailing chip (narrower than the pill)
+  // that would actually have fitted, leaving "+N" beside empty space.
+  if ((chipRights[n - 1] ?? 0) <= avail) return n;
+  let count = n;
+  for (let i = 0; i < n; i++) {
+    const right = chipRights[i] ?? 0;
+    const moreAfter = i < n - 1;
+    if (right > avail - (moreAfter ? pillW + gap : 0)) {
+      count = i;
+      break;
+    }
+  }
+  return count;
+}

frontend/src/lib/useChipOverflowFit.dom.test.tsx

@@ -0,0 +1,102 @@
+// @vitest-environment jsdom
+//
+// The shared chip-overflow measure core (S3). jsdom has no layout engine — every offset* / clientWidth
+// is 0 — so we stub those on the real ref'd nodes and assert the hook (a) reads the geometry off them
+// (chip right edges, pill width, the gap between the last chip and the pill, and the lane's available
+// width), (b) returns the caller's `decide` result, and (c) re-measures when `deps` changes.
+import { describe, it, expect, afterEach } from "vitest";
+import { useRef } from "react";
+import { render, screen, cleanup, act } from "@testing-library/react";
+import { useChipOverflowFit, type ChipRowGeometry } from "./useChipOverflowFit";
+
+function stub(
+  el: HTMLElement,
+  props: Partial<Record<"offsetLeft" | "offsetWidth" | "clientWidth", number>>,
+) {
+  for (const [k, v] of Object.entries(props)) {
+    Object.defineProperty(el, k, { configurable: true, get: () => v });
+  }
+}
+
+let lastGeom: ChipRowGeometry | null = null;
+
+function Harness({ deps }: { deps: string }) {
+  const rowRef = useRef<HTMLDivElement | null>(null);
+  const chipRefs = useRef<Array<HTMLElement | null>>([]);
+  const pillRef = useRef<HTMLSpanElement | null>(null);
+  const visible = useChipOverflowFit<number>({
+    rowRef,
+    chipsRef: chipRefs,
+    pillRef,
+    deps,
+    initial: -1,
+    decide: (g) => {
+      lastGeom = g;
+      // A trivial policy: count the chips whose right edge fits the lane.
+      return g.chipRights.filter((r) => r <= g.avail).length;
+    },
+  });
+  return (
+    <div>
+      <div ref={rowRef} data-testid="row" />
+      <span
+        ref={(el) => {
+          chipRefs.current[0] = el;
+        }}
+        data-testid="chip0"
+      />
+      <span
+        ref={(el) => {
+          chipRefs.current[1] = el;
+        }}
+        data-testid="chip1"
+      />
+      <span ref={pillRef} data-testid="pill" />
+      <div data-testid="visible">{visible}</div>
+    </div>
+  );
+}
+
+afterEach(() => {
+  cleanup();
+  lastGeom = null;
+});
+
+describe("useChipOverflowFit", () => {
+  it("reads geometry off the refs and returns the decide() result; re-measures on deps change", () => {
+    const { rerender } = render(<Harness deps="a" />);
+
+    // First pass ran against jsdom's 0-sized boxes.
+    expect(lastGeom).toEqual({ avail: 0, chipRights: [0, 0], pillW: 0, gap: 0 });
+
+    // Give the boxes real geometry, then bump deps to force a re-measure.
+    stub(screen.getByTestId("row"), { clientWidth: 100 });
+    stub(screen.getByTestId("chip0"), { offsetLeft: 0, offsetWidth: 30 }); // right edge 30
+    stub(screen.getByTestId("chip1"), { offsetLeft: 36, offsetWidth: 30 }); // right edge 66
+    stub(screen.getByTestId("pill"), { offsetLeft: 72, offsetWidth: 20 }); // gap = 72 - 66 = 6
+
+    act(() => {
+      rerender(<Harness deps="b" />);
+    });
+
+    expect(lastGeom).toEqual({ avail: 100, chipRights: [30, 66], pillW: 20, gap: 6 });
+    // decide counted both chips (30 and 66 both <= 100).
+    expect(screen.getByTestId("visible").textContent).toBe("2");
+  });
+
+  it("drops a chip whose right edge exceeds the lane, via the caller's policy", () => {
+    const { rerender } = render(<Harness deps="a" />);
+    stub(screen.getByTestId("row"), { clientWidth: 40 });
+    stub(screen.getByTestId("chip0"), { offsetLeft: 0, offsetWidth: 30 }); // right edge 30 <= 40
+    stub(screen.getByTestId("chip1"), { offsetLeft: 36, offsetWidth: 30 }); // right edge 66 > 40
+    stub(screen.getByTestId("pill"), { offsetLeft: 72, offsetWidth: 20 });
+
+    act(() => {
+      rerender(<Harness deps="b" />);
+    });
+
+    expect(lastGeom).toEqual({ avail: 40, chipRights: [30, 66], pillW: 20, gap: 6 });
+    // Only chip0 (right edge 30) fits the 40-wide lane; chip1 (66) is dropped by the policy.
+    expect(screen.getByTestId("visible").textContent).toBe("1");
+  });
+});

frontend/src/lib/useChipOverflowFit.ts

@@ -0,0 +1,79 @@
+import { useLayoutEffect, useRef, useState, type RefObject } from "react";
+
+// The shared DOM-measure-and-fit core for a single-line chip lane that collapses its overflow into a
+// trailing "+N" pill. Owns the layout-effect + ResizeObserver + initial-measure wiring and reads the
+// common geometry off a hidden mirror (each chip's right edge, the real rem gap, the pill width, and
+// the lane's available width); each caller layers its OWN fit policy in `decide` and gets that result
+// back. This was hand-rolled twice — the feed's applied-filter row (ActiveFilterChips, whose policy
+// also reserves a leading label + a trailing "Clear all" and can collapse to a single count pill) and
+// the channel tag cell (ChannelTagsControl, chips-only) — with the same measurement half copied in
+// both. The policy halves live in lib/chipOverflow.ts; this owns the measurement half.
+//
+// Coordinate note: `chipRights` are offsets within the hidden mirror (its children's offsetParent),
+// while `avail` is the VISIBLE lane's clientWidth — the two are comparable because the mirror is laid
+// out `absolute left-0 top-0` over the lane's start, so a mirror-relative right edge is the position a
+// chip would take in the real lane. Reads real rem boxes, so nothing is frozen in px (the text-size
+// slider scales the gap and chip widths, and the fit follows).
+
+export interface ChipRowGeometry {
+  /** Available lane width — the visible row's clientWidth. */
+  avail: number;
+  /** Right edge (offsetLeft + offsetWidth) of each chip in the measurer, index-aligned to the chips. */
+  chipRights: number[];
+  /** Width of the trailing sample "+N" pill in the measurer. */
+  pillW: number;
+  /** The real rendered inter-item gap (rem-scaled), read off the box between the last chip and the pill. */
+  gap: number;
+}
+
+export function useChipOverflowFit<R>(args: {
+  /** The VISIBLE lane; its clientWidth is the available width, and resize is observed on it. */
+  rowRef: RefObject<HTMLElement | null>;
+  /** The measurer's chip elements, index-aligned (a hidden mirror of the full chip set). */
+  chipsRef: RefObject<Array<HTMLElement | null>>;
+  /** The sample "+N" pill in the measurer. */
+  pillRef: RefObject<HTMLElement | null>;
+  /** Re-measure when this changes (a key over the chip set — count + labels, so a rename re-fits too). */
+  deps: unknown;
+  /** The caller's fit policy over the measured geometry. */
+  decide: (geom: ChipRowGeometry) => R;
+  /** Value used until the first measure (and while the refs aren't mounted). */
+  initial: R;
+}): R {
+  const { rowRef, chipsRef, pillRef, deps, decide, initial } = args;
+  const [result, setResult] = useState<R>(initial);
+  // Read `decide` fresh each run without making it an effect dependency: the effect re-runs on `deps`
+  // (and resize), and a caller's decide closure is rebuilt every render — depending on it would
+  // resubscribe the ResizeObserver on every render.
+  const decideRef = useRef(decide);
+  decideRef.current = decide;
+
+  useLayoutEffect(() => {
+    const row = rowRef.current;
+    const pill = pillRef.current;
+    if (!row || !pill) return;
+    const rightOf = (el: HTMLElement | null | undefined) =>
+      el ? el.offsetLeft + el.offsetWidth : 0;
+    const run = () => {
+      const chips = (chipsRef.current ?? []).filter((c): c is HTMLElement => c != null);
+      const lastChip = chips[chips.length - 1];
+      setResult(
+        decideRef.current({
+          avail: row.clientWidth,
+          chipRights: chips.map(rightOf),
+          pillW: pill.offsetWidth,
+          gap: lastChip ? Math.max(0, pill.offsetLeft - rightOf(lastChip)) : 0,
+        }),
+      );
+    };
+    run();
+    // jsdom has no ResizeObserver — the one-shot measure above still runs there (unit tests drive it).
+    if (typeof ResizeObserver === "undefined") return;
+    const ro = new ResizeObserver(run);
+    ro.observe(row);
+    return () => ro.disconnect();
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [deps]);
+
+  return result;
+}