3 changed files with 329 additions and 5 deletions
--- a/src/components/shared/CategoryCombobox.tsx
+++ b/src/components/shared/CategoryCombobox.tsx
@ -1,5 +1,13 @@
-import { useState, useRef, useEffect, useCallback, useId, useMemo } from "react";
+import { useState, useRef, useEffect, useLayoutEffect, useCallback, useId, useMemo } from "react";
+import { createPortal } from "react-dom";
 import { useTranslation } from "react-i18next";
+import {
+  LISTBOX_MAX_HEIGHT,
+  followAnchor,
+  placeListbox,
+  samePlacement,
+  type ListboxPlacement,
+} from "./placeListbox";
 /**
 * Minimal shape the combobox needs. `Category` (transactions, reports) and
 * `CategoryTreeNode` (categories page) both satisfy it, so callers pass their
@ -135,6 +143,7 @@ export default function CategoryCombobox<T extends CategoryLike = CategoryLike>(
  const [open, setOpen] = useState(false);
  const [query, setQuery] = useState("");
  const [highlightIndex, setHighlightIndex] = useState(0);
+  const [placement, setPlacement] = useState<ListboxPlacement | null>(null);
  const inputRef = useRef<HTMLInputElement>(null);
  const listRef = useRef<HTMLUListElement>(null);
  const containerRef = useRef<HTMLDivElement>(null);
@ -187,10 +196,36 @@ export default function CategoryCombobox<T extends CategoryLike = CategoryLike>(
    }
  }, [highlightIndex, open]);

+  // The open list lives in a portal on document.body (#429): no scrolling
+  // container clips it. It is placed on its input before the first paint,
+  // then follows the input on every scroll and resize, even out of the
+  // visible part of its container; the listeners go with the list.
+  const updatePlacement = useCallback(() => {
+    const input = inputRef.current;
+    if (!input) return;
+    const next = placeListbox({
+      anchor: input.getBoundingClientRect(),
+      viewportHeight: window.innerHeight,
+    });
+    setPlacement((prev) => (samePlacement(prev, next) ? prev : next));
+  }, []);
+
+  useLayoutEffect(() => {
+    if (!open) return;
+    updatePlacement();
+    return followAnchor(window, updatePlacement, () => listRef.current);
+  }, [open, updatePlacement]);
+
  useEffect(() => {
    if (!open) return;
    const handler = (e: MouseEvent) => {
-      if (containerRef.current && !containerRef.current.contains(e.target as Node)) {
+      // The list is outside the container in the DOM: a press on an option
+      // counts as inside, or the list would close before the click selects.
+      if (
+        containerRef.current &&
+        !containerRef.current.contains(e.target as Node) &&
+        !listRef.current?.contains(e.target as Node)
+      ) {
        setOpen(false);
        setQuery("");
      }
@ -277,12 +312,20 @@ export default function CategoryCombobox<T extends CategoryLike = CategoryLike>(
        onKeyDown={handleKeyDown}
        className={`w-full ${px} ${py} text-sm rounded-lg border border-[var(--border)] bg-[var(--background)] text-[var(--foreground)] placeholder:text-[var(--muted-foreground)] focus:outline-none focus:ring-1 focus:ring-[var(--primary)]`}
      />
-      {open && totalItems > 0 && (
+      {open && totalItems > 0 && placement && createPortal(
        <ul
          ref={listRef}
          id={listboxId}
          role="listbox"
-          className="absolute z-50 mt-1 max-h-60 w-full overflow-auto rounded-lg border border-[var(--border)] bg-[var(--card)] shadow-lg"
+          // z-[300]: above every window of the app, the highest at z-[210].
+          className="fixed z-[300] overflow-auto rounded-lg border border-[var(--border)] bg-[var(--card)] shadow-lg"
+          style={{
+            top: placement.top,
+            bottom: placement.bottom,
+            left: placement.left,
+            width: placement.width,
+            maxHeight: LISTBOX_MAX_HEIGHT,
+          }}
        >
          {filteredExtras.map((opt, i) => (
            <li
@ -327,7 +370,8 @@ export default function CategoryCombobox<T extends CategoryLike = CategoryLike>(
              </li>
            );
          })}
-        </ul>
+        </ul>,
+        document.body,
      )}
    </div>
  );
--- a/src/components/shared/placeListbox.test.ts
+++ b/src/components/shared/placeListbox.test.ts
@ -0,0 +1,175 @@
+// placeListbox — where the open list of a category combobox goes (#429).
+// The portal itself needs a DOM (manual check); its placement rule and the
+// listeners that keep it on its input do not, and are pinned here.
+import { describe, it, expect, vi } from "vitest";
+import {
+  LISTBOX_GAP,
+  LISTBOX_MAX_HEIGHT,
+  followAnchor,
+  placeListbox,
+  samePlacement,
+  type ListenerTarget,
+  type ListboxPlacement,
+} from "./placeListbox";
+
+/** An input 30 px high and 200 px wide, its top at `top`. */
+const anchorAt = (top: number) => ({ top, bottom: top + 30, left: 50, width: 200 });
+
+describe("placeListbox", () => {
+  it("keeps the list's size of max-h-60 and the gap of mt-1", () => {
+    expect(LISTBOX_MAX_HEIGHT).toBe(240);
+    expect(LISTBOX_GAP).toBe(4);
+  });
+
+  it("places the list under the input, as wide as it, at the input's left edge", () => {
+    expect(placeListbox({ anchor: anchorAt(100), viewportHeight: 800 })).toEqual({
+      left: 50,
+      width: 200,
+      openUpward: false,
+      top: 134,
+    });
+  });
+
+  it("stays below when the room below is exactly the list's maximum height", () => {
+    // 130 (input's bottom) + 4 + 240 = 374.
+    const placement = placeListbox({ anchor: anchorAt(100), viewportHeight: 374 });
+    expect(placement.openUpward).toBe(false);
+    expect(placement.top).toBe(134);
+  });
+
+  it("opens upward when the room below is short and the room above suffices", () => {
+    const placement = placeListbox({ anchor: anchorAt(700), viewportHeight: 800 });
+    expect(placement).toEqual({ left: 50, width: 200, openUpward: true, bottom: 104 });
+    // Its bottom edge sits the gap above the input, whatever its height.
+    expect(800 - placement.bottom!).toBe(700 - LISTBOX_GAP);
+  });
+
+  it("opens upward when the room above is exactly the list's maximum height", () => {
+    // Room above: 244 - 4 = 240; room below: 300 - 274 - 4 = 22.
+    expect(placeListbox({ anchor: anchorAt(244), viewportHeight: 300 }).openUpward).toBe(true);
+  });
+
+  it("stays below when room is short on both sides, even with more room above", () => {
+    // Room above: 200 - 4 = 196; room below: 400 - 230 - 4 = 166.
+    expect(placeListbox({ anchor: anchorAt(200), viewportHeight: 400 })).toEqual({
+      left: 50,
+      width: 200,
+      openUpward: false,
+      top: 234,
+    });
+    // One pixel short above: still below.
+    expect(placeListbox({ anchor: anchorAt(243), viewportHeight: 300 }).openUpward).toBe(false);
+  });
+
+  it("reads the maximum height and the gap it is given", () => {
+    // Room below: 800 - 730 - 10 = 60, under 100; room above: 700 - 10 = 690.
+    expect(
+      placeListbox({ anchor: anchorAt(700), viewportHeight: 800, maxHeight: 100, gap: 10 }),
+    ).toEqual({ left: 50, width: 200, openUpward: true, bottom: 110 });
+    // Room below: 60, enough for a list of 50 px.
+    expect(
+      placeListbox({ anchor: anchorAt(700), viewportHeight: 800, maxHeight: 50, gap: 10 }),
+    ).toEqual({ left: 50, width: 200, openUpward: false, top: 740 });
+  });
+});
+
+describe("samePlacement", () => {
+  const below: ListboxPlacement = { left: 50, width: 200, openUpward: false, top: 134 };
+
+  it("tells an unchanged position apart from a moved one", () => {
+    expect(samePlacement({ ...below }, below)).toBe(true);
+    expect(samePlacement(null, below)).toBe(false);
+    expect(samePlacement({ ...below, top: 135 }, below)).toBe(false);
+    expect(samePlacement({ ...below, left: 51 }, below)).toBe(false);
+    expect(samePlacement({ ...below, width: 201 }, below)).toBe(false);
+    expect(
+      samePlacement({ left: 50, width: 200, openUpward: true, bottom: 134 }, below),
+    ).toBe(false);
+  });
+});
+
+/**
+ * A fake `window` with the DOM's rule: a listener is removed only by the same
+ * type, the same function and the same capture flag.
+ */
+function fakeTarget() {
+  const listeners: Array<{ type: string; fn: (event: Event) => void; capture: boolean }> = [];
+  const target: ListenerTarget = {
+    addEventListener: (type, fn, capture = false) => {
+      listeners.push({ type, fn, capture });
+    },
+    removeEventListener: (type, fn, capture = false) => {
+      const at = listeners.findIndex((l) => l.type === type && l.fn === fn && l.capture === capture);
+      if (at >= 0) listeners.splice(at, 1);
+    },
+  };
+  const fire = (type: string, eventTarget: unknown = null) => {
+    for (const l of [...listeners]) if (l.type === type) l.fn({ target: eventTarget } as unknown as Event);
+  };
+  return { target, listeners, fire };
+}
+
+/** A node of the list: it contains itself and its options. */
+const option = { id: "option" };
+const listNode = { contains: (node: unknown) => node === listNode || node === option } as unknown as Node;
+const outside = { id: "a scrolling container" };
+
+describe("followAnchor", () => {
+  it("listens to scroll in the capture phase, since scroll does not bubble, and to resize", () => {
+    const { target, listeners } = fakeTarget();
+    followAnchor(target, vi.fn(), () => listNode);
+    expect(listeners.map(({ type, capture }) => ({ type, capture }))).toEqual([
+      { type: "scroll", capture: true },
+      { type: "resize", capture: false },
+    ]);
+  });
+
+  it("moves the list on a scroll of the page or of a container, and on a resize", () => {
+    const { target, fire } = fakeTarget();
+    const onMove = vi.fn();
+    followAnchor(target, onMove, () => listNode);
+    fire("scroll", outside);
+    expect(onMove).toHaveBeenCalledTimes(1);
+    fire("resize");
+    expect(onMove).toHaveBeenCalledTimes(2);
+  });
+
+  it("does not move the list on a scroll of the list itself", () => {
+    const { target, fire } = fakeTarget();
+    const onMove = vi.fn();
+    followAnchor(target, onMove, () => listNode);
+    fire("scroll", listNode);
+    fire("scroll", option);
+    expect(onMove).not.toHaveBeenCalled();
+    // Before the list is in the page, every scroll counts.
+    const early = fakeTarget();
+    const onEarlyMove = vi.fn();
+    followAnchor(early.target, onEarlyMove, () => null);
+    early.fire("scroll", outside);
+    expect(onEarlyMove).toHaveBeenCalledTimes(1);
+  });
+
+  it("removes exactly its own listeners, capture flag included", () => {
+    const { target, listeners, fire } = fakeTarget();
+    const onMove = vi.fn();
+    const stop = followAnchor(target, onMove, () => listNode);
+    stop();
+    expect(listeners).toEqual([]);
+    fire("scroll", outside);
+    fire("resize");
+    expect(onMove).not.toHaveBeenCalled();
+  });
+
+  it("leaves another open list's listeners in place", () => {
+    const { target, listeners, fire } = fakeTarget();
+    const first = vi.fn();
+    const second = vi.fn();
+    const stopFirst = followAnchor(target, first, () => listNode);
+    followAnchor(target, second, () => listNode);
+    stopFirst();
+    expect(listeners).toHaveLength(2);
+    fire("scroll", outside);
+    expect(first).not.toHaveBeenCalled();
+    expect(second).toHaveBeenCalledTimes(1);
+  });
+});
--- a/src/components/shared/placeListbox.ts
+++ b/src/components/shared/placeListbox.ts
@ -0,0 +1,105 @@
+// placeListbox — where the open list of a combobox goes (#429).
+//
+// `CategoryCombobox` renders its open list in a portal on `document.body`,
+// with `position: fixed`, so no scrolling container clips it: the body of the
+// split window, the wrapper of the Transactions table, a modal. What does not
+// need a DOM lives here, so vitest's node environment can pin it: the
+// placement rule, and the listeners that keep the list on its input.
+
+/** Maximum height of the list, in pixels (Tailwind's `max-h-60`). */
+export const LISTBOX_MAX_HEIGHT = 240;
+
+/** Gap between the input and the list, in pixels (the former `mt-1`). */
+export const LISTBOX_GAP = 4;
+
+/** What the placement reads of the input: a `DOMRect` fits. */
+export interface ListboxAnchor {
+  top: number;
+  bottom: number;
+  left: number;
+  width: number;
+}
+
+/**
+ * Fixed position of the list, in viewport pixels, as wide as the input.
+ * Below the input, `top` is the list's top edge. Above it, `bottom` is the
+ * distance from the viewport's bottom edge to the list's bottom edge, so a
+ * list shorter than its maximum height still ends right above the input.
+ */
+export interface ListboxPlacement {
+  left: number;
+  width: number;
+  openUpward: boolean;
+  top?: number;
+  bottom?: number;
+}
+
+export interface PlaceListboxInput {
+  anchor: ListboxAnchor;
+  viewportHeight: number;
+  maxHeight?: number;
+  gap?: number;
+}
+
+/**
+ * Under the input, unless the list does not fit below and does fit above:
+ * then above. With room on neither side, it stays below, its usual place.
+ */
+export function placeListbox({
+  anchor,
+  viewportHeight,
+  maxHeight = LISTBOX_MAX_HEIGHT,
+  gap = LISTBOX_GAP,
+}: PlaceListboxInput): ListboxPlacement {
+  const spaceBelow = viewportHeight - anchor.bottom - gap;
+  const spaceAbove = anchor.top - gap;
+  const base = { left: anchor.left, width: anchor.width };
+  if (spaceBelow < maxHeight && spaceAbove >= maxHeight) {
+    return { ...base, openUpward: true, bottom: viewportHeight - anchor.top + gap };
+  }
+  return { ...base, openUpward: false, top: anchor.bottom + gap };
+}
+
+/** Same position: a scroll that does not move the input renders nothing again. */
+export function samePlacement(a: ListboxPlacement | null, b: ListboxPlacement): boolean {
+  return (
+    a !== null &&
+    a.left === b.left &&
+    a.width === b.width &&
+    a.openUpward === b.openUpward &&
+    a.top === b.top &&
+    a.bottom === b.bottom
+  );
+}
+
+/** The part of `window` the listeners use, so a test can pass a fake. */
+export interface ListenerTarget {
+  addEventListener(type: string, listener: (event: Event) => void, capture?: boolean): void;
+  removeEventListener(type: string, listener: (event: Event) => void, capture?: boolean): void;
+}
+
+/**
+ * Keeps the list on its input while it is open: `onMove` runs on every resize
+ * and on every scroll, of the page or of any scrolling container (a modal's
+ * body, the page's main area), hence the capture phase, since `scroll` does
+ * not bubble. A scroll of the list itself is skipped: its options moving do
+ * not move the input. Returns the cleanup, which removes exactly these
+ * listeners, capture flag included.
+ */
+export function followAnchor(
+  target: ListenerTarget,
+  onMove: () => void,
+  list: () => Pick<Node, "contains"> | null,
+): () => void {
+  const onScroll = (event: Event) => {
+    if (list()?.contains(event.target as Node | null)) return;
+    onMove();
+  };
+  const onResize = () => onMove();
+  target.addEventListener("scroll", onScroll, true);
+  target.addEventListener("resize", onResize);
+  return () => {
+    target.removeEventListener("scroll", onScroll, true);
+    target.removeEventListener("resize", onResize);
+  };
+}