Replace ActionBar overflow calculations with CSS wrapping approach

.changeset/many-suns-promise.md

@@ -0,0 +1,5 @@
+---
+"@primer/react": minor
+---
+
+Replace `ActionBar` overflow calculations with CSS wrapping approach to improve performance and stability

e2e/components/drafts/ActionBar.test.ts

@@ -47,12 +47,12 @@ test.describe('ActionBar', () => {
             },
           })
           const toolbarButtonSelector = `button[data-component="IconButton"]`
-          await expect(page.locator(toolbarButtonSelector)).toHaveCount(10)
+          await expect(page.locator(toolbarButtonSelector).filter({visible: true})).toHaveCount(10)
           await page.setViewportSize({width: viewports['primer.breakpoint.xs'], height: 768})
           await page.getByLabel('Task List').waitFor({
             state: 'hidden',
           })
-          await expect(page.locator(toolbarButtonSelector)).toHaveCount(8)
+          await expect(page.locator(toolbarButtonSelector).filter({visible: true})).toHaveCount(8)
           const moreButtonSelector = page.getByLabel('More Comment box toolbar items')
           await moreButtonSelector.click()
           await expect(page.locator('ul[role="menu"] [role="menuitem"]')).toHaveCount(3)

packages/react/src/ActionBar/ActionBar.module.css

@@ -6,10 +6,34 @@
   /* wonder why this is here */
   /* stylelint-disable-next-line primer/spacing */
   margin-bottom: -1px;
-  white-space: nowrap;
   list-style: none;
-  align-items: center;
+  align-items: flex-start;
   gap: var(--actionbar-gap, var(--stack-gap-condensed));
+  overflow: hidden;
+  /* Explicit height is required to clip wrapped items */
+  height: var(--actionbar-height, var(--control-small-size));
+
+  /* Only apply scroll animation before overflow is calculated (for SSR/initial render) */
+  &:not([data-has-overflow]) {
+    /* Scroll-based animations have no effect unless the container is scrollable (has overflow, even with overflow:hidden)
+    so we can use them to detect overflow. It would be cleaner to use scroll-state container queries for this, but
+    browser support for scroll-driven animations is slightly better. */
+    animation: detect-overflow linear;
+    animation-timeline: scroll(self block);
+  }
+
+  /* After initial render, JS is used to control visibility which provides progressive enhancement for unsupported browsers */
+  &[data-has-overflow='true'] {
+    --morebutton-display: block;
+  }
+
+  &:where([data-size='medium']) {
+    --actionbar-height: var(--control-medium-size);
+  }
+
+  &:where([data-size='large']) {
+    --actionbar-height: var(--control-large-size);
+  }
 
   /* Gap scale (mirrors Stack) */
   &:where([data-gap='none']) {
@@ -23,6 +47,20 @@
   &:where([data-gap='condensed']) {
     --actionbar-gap: var(--stack-gap-condensed);
   }
+
+  & [data-overflowing] {
+    /* Hide overflowing items. Even though they are clipped by `overflow: hidden`, setting `visibility: hidden` ensures
+    they can't accidentally be shown and also hides them from screen readers / keyboard nav. `!important` prevents
+    consumers from unintentionally overriding this and breaking accessibility. */
+    visibility: hidden !important;
+  }
+}
+
+@keyframes detect-overflow {
+  0%,
+  100% {
+    --morebutton-display: block;
+  }
 }
 
 .Nav {
@@ -44,10 +82,24 @@
     content: '';
     /* stylelint-disable-next-line primer/colors */
     background: var(--borderColor-muted);
+    /* stylelint-disable-next-line primer/spacing */
+    margin-top: calc((var(--actionbar-height) - var(--base-size-20)) / 2);
   }
 }
 
 .Group {
   display: flex;
   gap: inherit;
 }
+
+.OverflowContainer {
+  display: flex;
+  flex-wrap: wrap;
+  gap: inherit;
+  justify-content: flex-end;
+  overflow: hidden;
+}
+
+.MoreButton {
+  display: var(--morebutton-display, none);
+}

packages/react/src/ActionBar/ActionBar.test.tsx

@@ -228,13 +228,14 @@ describe('ActionBar Registry System', () => {
     render(
       <div style={{width: 0, overflow: 'hidden'}}>
         <ActionBar aria-label="Zero width">
-          <ActionBar.IconButton icon={BoldIcon} aria-label="Zero width button" />
+          <ActionBar.IconButton icon={BoldIcon} aria-label="Zero width button" data-testid="zero-width-button" />
         </ActionBar>
       </div>,
     )
 
     // Component should still render even with zero width
-    expect(screen.getByRole('button', {name: 'Zero width button'})).toBeInTheDocument()
+    // Button is unlabeled because the label is hidden, so we select by test id instead
+    expect(screen.getByTestId('zero-width-button')).toBeInTheDocument()
   })
 
   it('should clean up registry on unmount', async () => {