feat: add usePopoverPosition composable

Replaces reka-ui's floating-position logic for the Select listbox with
a simple flip-above/below fixed-position calculation.

packages/library/src/lib/use-popover-position.ts

@@ -0,0 +1,70 @@
+import type { Ref } from 'vue'
+import { nextTick, onUnmounted, reactive, watch } from 'vue'
+
+export interface PopoverPosition {
+  top: number
+  left: number
+  minWidth: number
+  maxHeight: number
+  side: 'top' | 'bottom'
+}
+
+const GAP = 4
+const VIEWPORT_MARGIN = 8
+
+export function usePopoverPosition(
+  triggerRef: Ref<HTMLElement | null>,
+  contentRef: Ref<HTMLElement | null>,
+  open: Ref<boolean>,
+) {
+  const position = reactive<PopoverPosition>({ top: 0, left: 0, minWidth: 0, maxHeight: 0, side: 'bottom' })
+
+  function update() {
+    const trigger = triggerRef.value
+    const content = contentRef.value
+    if (!trigger || !content) return
+
+    const triggerRect = trigger.getBoundingClientRect()
+    const contentHeight = content.offsetHeight
+    const spaceBelow = window.innerHeight - triggerRect.bottom - VIEWPORT_MARGIN
+    const spaceAbove = triggerRect.top - VIEWPORT_MARGIN
+    const placeAbove = spaceBelow < contentHeight && spaceAbove > spaceBelow
+
+    position.side = placeAbove ? 'top' : 'bottom'
+    position.left = Math.min(
+      Math.max(triggerRect.left, VIEWPORT_MARGIN),
+      window.innerWidth - triggerRect.width - VIEWPORT_MARGIN,
+    )
+    position.minWidth = triggerRect.width
+    position.maxHeight = Math.max(placeAbove ? spaceAbove : spaceBelow, 0)
+    position.top = placeAbove
+      ? triggerRect.top - GAP - Math.min(contentHeight, position.maxHeight)
+      : triggerRect.bottom + GAP
+  }
+
+  function handleReposition() {
+    if (open.value) update()
+  }
+
+  watch(
+    open,
+    (isOpen) => {
+      if (isOpen) {
+        nextTick(update)
+        window.addEventListener('scroll', handleReposition, true)
+        window.addEventListener('resize', handleReposition)
+      } else {
+        window.removeEventListener('scroll', handleReposition, true)
+        window.removeEventListener('resize', handleReposition)
+      }
+    },
+    { immediate: true },
+  )
+
+  onUnmounted(() => {
+    window.removeEventListener('scroll', handleReposition, true)
+    window.removeEventListener('resize', handleReposition)
+  })
+
+  return position
+}

packages/library/tests/use-popover-position.spec.ts

@@ -0,0 +1,57 @@
+import { describe, it, expect, vi, afterEach } from 'vitest'
+import { nextTick, ref } from 'vue'
+import { usePopoverPosition } from '../src/lib/use-popover-position'
+
+function mockRect(el: HTMLElement, rect: Partial<DOMRect>) {
+  vi.spyOn(el, 'getBoundingClientRect').mockReturnValue({
+    top: 0, left: 0, right: 0, bottom: 0, width: 0, height: 0, x: 0, y: 0,
+    toJSON: () => {},
+    ...rect,
+  } as DOMRect)
+}
+
+describe('usePopoverPosition', () => {
+  afterEach(() => {
+    document.body.replaceChildren()
+    vi.restoreAllMocks()
+  })
+
+  it('places the content below the trigger when there is enough room', async () => {
+    const trigger = document.createElement('button')
+    const content = document.createElement('div')
+    document.body.append(trigger, content)
+    mockRect(trigger, { top: 100, bottom: 130, left: 20, width: 80 })
+    Object.defineProperty(content, 'offsetHeight', { value: 40, configurable: true })
+    vi.spyOn(window, 'innerHeight', 'get').mockReturnValue(800)
+    vi.spyOn(window, 'innerWidth', 'get').mockReturnValue(1000)
+
+    const open = ref(false)
+    const position = usePopoverPosition(ref(trigger), ref(content), open)
+    open.value = true
+    await nextTick()
+    await nextTick()
+
+    expect(position.side).toBe('bottom')
+    expect(position.top).toBe(134)
+    expect(position.minWidth).toBe(80)
+  })
+
+  it('flips above the trigger when there is not enough room below', async () => {
+    const trigger = document.createElement('button')
+    const content = document.createElement('div')
+    document.body.append(trigger, content)
+    mockRect(trigger, { top: 700, bottom: 730, left: 20, width: 80 })
+    Object.defineProperty(content, 'offsetHeight', { value: 200, configurable: true })
+    vi.spyOn(window, 'innerHeight', 'get').mockReturnValue(800)
+    vi.spyOn(window, 'innerWidth', 'get').mockReturnValue(1000)
+
+    const open = ref(false)
+    const position = usePopoverPosition(ref(trigger), ref(content), open)
+    open.value = true
+    await nextTick()
+    await nextTick()
+
+    expect(position.side).toBe('top')
+    expect(position.top).toBeLessThan(700)
+  })
+})