perf: virtualize FormDropdownMenu to reduce DOM nodes and image requests (#8476)

## Summary

Virtualize the FormDropdownMenu to only render visible items, fixing
slow dropdown performance on cloud.

## Changes

- Integrate `VirtualGrid` into `FormDropdownMenu` for virtualized
rendering
- Add computed properties for grid configuration per layout mode
(grid/list/list-small)
- Extend `VirtualGrid` slot to provide original item index for O(1)
lookups
- Change container from `max-h-[640px]` to fixed `h-[640px]` for proper
virtualization

## Review Focus

- VirtualGrid integration within the popover context
- Layout mode switching with `:key="layoutMode"` to force re-render
- Grid style computed properties match original Tailwind classes

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8476-perf-virtualize-FormDropdownMenu-to-reduce-DOM-nodes-and-image-requests-2f86d73d365081b3a79dd5e0b84df944)
by [Unito](https://www.unito.io)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Dropdowns now render with a virtualized grid/list (stable indexes,
responsive sizing) and show an empty-state icon when no items exist.

* **Bug Fixes**
* Reduced layout shift and rendering glitches with improved
spacer/scroll calculations and more reliable media measurement.

* **Style**
* Simplified media rendering (standard img/video), unified item visuals
and hover/background behavior.

* **Tests**
* Added unit and end-to-end tests for virtualization, indexing, layouts,
dynamic updates, and empty states.

* **Breaking Changes**
* Dropdown item/selection shapes and related component props/events were
updated (adapter changes may be required).
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: GitHub Action <action@github.com>

src/components/common/VirtualGrid.test.ts

@@ -0,0 +1,189 @@
+import { mount } from '@vue/test-utils'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import type { Ref } from 'vue'
+import { nextTick, ref } from 'vue'
+
+import VirtualGrid from './VirtualGrid.vue'
+
+type TestItem = { key: string; name: string }
+
+let mockedWidth: Ref<number>
+let mockedHeight: Ref<number>
+let mockedScrollY: Ref<number>
+
+vi.mock('@vueuse/core', async () => {
+  const actual = await vi.importActual<Record<string, unknown>>('@vueuse/core')
+  return {
+    ...actual,
+    useElementSize: () => ({ width: mockedWidth, height: mockedHeight }),
+    useScroll: () => ({ y: mockedScrollY })
+  }
+})
+
+beforeEach(() => {
+  mockedWidth = ref(400)
+  mockedHeight = ref(200)
+  mockedScrollY = ref(0)
+})
+
+function createItems(count: number): TestItem[] {
+  return Array.from({ length: count }, (_, i) => ({
+    key: `item-${i}`,
+    name: `Item ${i}`
+  }))
+}
+
+describe('VirtualGrid', () => {
+  const defaultGridStyle = {
+    display: 'grid',
+    gridTemplateColumns: 'repeat(4, 1fr)',
+    gap: '1rem'
+  }
+
+  it('renders items within the visible range', async () => {
+    const items = createItems(100)
+    mockedWidth.value = 400
+    mockedHeight.value = 200
+    mockedScrollY.value = 0
+
+    const wrapper = mount(VirtualGrid<TestItem>, {
+      props: {
+        items,
+        gridStyle: defaultGridStyle,
+        defaultItemHeight: 100,
+        defaultItemWidth: 100,
+        maxColumns: 4,
+        bufferRows: 1
+      },
+      slots: {
+        item: `<template #item="{ item }">
+          <div class="test-item">{{ item.name }}</div>
+        </template>`
+      },
+      attachTo: document.body
+    })
+
+    await nextTick()
+
+    const renderedItems = wrapper.findAll('.test-item')
+    expect(renderedItems.length).toBeGreaterThan(0)
+    expect(renderedItems.length).toBeLessThan(items.length)
+
+    wrapper.unmount()
+  })
+
+  it('provides correct index in slot props', async () => {
+    const items = createItems(20)
+    const receivedIndices: number[] = []
+    mockedWidth.value = 400
+    mockedHeight.value = 200
+    mockedScrollY.value = 0
+
+    const wrapper = mount(VirtualGrid<TestItem>, {
+      props: {
+        items,
+        gridStyle: defaultGridStyle,
+        defaultItemHeight: 50,
+        defaultItemWidth: 100,
+        maxColumns: 1,
+        bufferRows: 0
+      },
+      slots: {
+        item: ({ index }: { index: number }) => {
+          receivedIndices.push(index)
+          return null
+        }
+      },
+      attachTo: document.body
+    })
+
+    await nextTick()
+
+    expect(receivedIndices.length).toBeGreaterThan(0)
+    expect(receivedIndices[0]).toBe(0)
+    for (let i = 1; i < receivedIndices.length; i++) {
+      expect(receivedIndices[i]).toBe(receivedIndices[i - 1] + 1)
+    }
+
+    wrapper.unmount()
+  })
+
+  it('respects maxColumns prop', async () => {
+    const items = createItems(10)
+    mockedWidth.value = 400
+    mockedHeight.value = 200
+    mockedScrollY.value = 0
+
+    const wrapper = mount(VirtualGrid<TestItem>, {
+      props: {
+        items,
+        gridStyle: defaultGridStyle,
+        maxColumns: 2
+      },
+      attachTo: document.body
+    })
+
+    await nextTick()
+
+    const gridElement = wrapper.find('[style*="display: grid"]')
+    expect(gridElement.exists()).toBe(true)
+
+    const gridEl = gridElement.element as HTMLElement
+    expect(gridEl.style.gridTemplateColumns).toBe('repeat(2, minmax(0, 1fr))')
+
+    wrapper.unmount()
+  })
+
+  it('renders empty when no items provided', async () => {
+    const wrapper = mount(VirtualGrid<TestItem>, {
+      props: {
+        items: [],
+        gridStyle: defaultGridStyle
+      },
+      slots: {
+        item: `<template #item="{ item }">
+          <div class="test-item">{{ item.name }}</div>
+        </template>`
+      }
+    })
+
+    await nextTick()
+
+    const renderedItems = wrapper.findAll('.test-item')
+    expect(renderedItems.length).toBe(0)
+
+    wrapper.unmount()
+  })
+
+  it('forces cols to maxColumns when maxColumns is finite', async () => {
+    mockedWidth.value = 100
+    mockedHeight.value = 200
+    mockedScrollY.value = 0
+
+    const items = createItems(20)
+    const wrapper = mount(VirtualGrid<TestItem>, {
+      props: {
+        items,
+        gridStyle: defaultGridStyle,
+        defaultItemHeight: 50,
+        defaultItemWidth: 200,
+        maxColumns: 4,
+        bufferRows: 0
+      },
+      slots: {
+        item: `<template #item="{ item }">
+          <div class="test-item">{{ item.name }}</div>
+        </template>`
+      },
+      attachTo: document.body
+    })
+
+    await nextTick()
+
+    const renderedItems = wrapper.findAll('.test-item')
+    expect(renderedItems.length).toBeGreaterThan(0)
+    expect(renderedItems.length % 4).toBe(0)
+
+    wrapper.unmount()
+  })
+})

src/components/common/VirtualGrid.vue

@@ -1,17 +1,16 @@
 <template>
   <div
     ref="container"
-    class="h-full overflow-y-auto scrollbar-thin scrollbar-track-transparent scrollbar-thumb-(--dialog-surface)"
+    class="h-full overflow-y-auto [overflow-anchor:none] [scrollbar-gutter:stable] scrollbar-thin scrollbar-track-transparent scrollbar-thumb-(--dialog-surface)"
   >
     <div :style="topSpacerStyle" />
     <div :style="mergedGridStyle">
       <div
-        v-for="item in renderedItems"
+        v-for="(item, i) in renderedItems"
         :key="item.key"
-        class="transition-[width] duration-150 ease-out"
         data-virtual-grid-item
       >
-        <slot name="item" :item="item" />
+        <slot name="item" :item :index="state.start + i" />
       </div>
     </div>
     <div :style="bottomSpacerStyle" />
@@ -66,9 +65,10 @@ const { y: scrollY } = useScroll(container, {
   eventListenerOptions: { passive: true }
 })
 
-const cols = computed(() =>
-  Math.min(Math.floor(width.value / itemWidth.value) || 1, maxColumns)
-)
+const cols = computed(() => {
+  if (maxColumns !== Infinity) return maxColumns
+  return Math.floor(width.value / itemWidth.value) || 1
+})
 
 const mergedGridStyle = computed<CSSProperties>(() => {
   if (maxColumns === Infinity) return gridStyle
@@ -101,8 +101,9 @@ const renderedItems = computed(() =>
   isValidGrid.value ? items.slice(state.value.start, state.value.end) : []
 )
 
-function rowsToHeight(rows: number): string {
-  return `${(rows / cols.value) * itemHeight.value}px`
+function rowsToHeight(itemsCount: number): string {
+  const rows = Math.ceil(itemsCount / cols.value)
+  return `${rows * itemHeight.value}px`
 }
 const topSpacerStyle = computed<CSSProperties>(() => ({
   height: rowsToHeight(state.value.start)
@@ -118,11 +119,10 @@ whenever(
   }
 )
 
-const updateItemSize = () => {
+function updateItemSize(): void {
   if (container.value) {
     const firstItem = container.value.querySelector('[data-virtual-grid-item]')
 
-    // Don't update item size if the first item is not rendered yet
     if (!firstItem?.clientHeight || !firstItem?.clientWidth) return
 
     if (itemHeight.value !== firstItem.clientHeight) {