Fix: Update tabs to remain the same size between unselected and selected states

packages/react-components/react-tabs/etc/react-tabs.api.md

@@ -44,7 +44,7 @@ export const TabList: ForwardRefComponent<TabListProps>;
 export const tabListClassNames: SlotClassNames<TabListSlots>;
 
 // @public (undocumented)
-export type TabListContextValue = Pick<TabListProps, 'onTabSelect' | 'selectedValue'> & Required<Pick<TabListProps, 'appearance' | 'disabled' | 'size' | 'vertical'>> & {
+export type TabListContextValue = Pick<TabListProps, 'onTabSelect' | 'selectedValue' | 'reserveSelectedTabSpace'> & Required<Pick<TabListProps, 'appearance' | 'disabled' | 'size' | 'vertical'>> & {
     onRegister: RegisterTabEventHandler;
     onUnregister: RegisterTabEventHandler;
     onSelect: SelectTabEventHandler;
@@ -63,6 +63,7 @@ export type TabListContextValues = {
 // @public
 export type TabListProps = ComponentProps<TabListSlots> & {
     appearance?: 'transparent' | 'subtle';
+    reserveSelectedTabSpace?: boolean;
     defaultSelectedValue?: TabValue;
     disabled?: boolean;
     onTabSelect?: SelectTabEventHandler;
@@ -103,6 +104,7 @@ export type TabState = ComponentState<TabSlots> & Pick<TabProps, 'value'> & Requ
     appearance?: 'transparent' | 'subtle';
     iconOnly: boolean;
     selected: boolean;
+    contentReservedSpaceClassName?: string;
     size: 'small' | 'medium';
     vertical: boolean;
 };

packages/react-components/react-tabs/src/components/Tab/Tab.types.ts

@@ -56,6 +56,11 @@ export type TabState = ComponentState<TabSlots> &
      * If this tab is selected
      */
     selected: boolean;
+    /**
+     * When defined, tab content with selected style is rendered hidden to reserve space.
+     * This keeps consistent content size between unselected and selected states.
+     */
+    contentReservedSpaceClassName?: string;
     /**
      * A tab can be either 'small' or 'medium' size.
      */

packages/react-components/react-tabs/src/components/Tab/renderTab.tsx

@@ -12,6 +12,9 @@ export const renderTab_unstable = (state: TabState) => {
     <slots.root {...slotProps.root}>
       {slots.icon && <slots.icon {...slotProps.icon} />}
       {!state.iconOnly && <slots.content {...slotProps.content} />}
+      {!state.selected && !state.iconOnly && state.contentReservedSpaceClassName !== undefined && (
+        <slots.content {...slotProps.content} className={state.contentReservedSpaceClassName} />
+      )}
     </slots.root>
   );
 };

packages/react-components/react-tabs/src/components/Tab/useTab.ts

@@ -18,6 +18,7 @@ export const useTab_unstable = (props: TabProps, ref: React.Ref<HTMLElement>): T
   const { content, disabled: tabDisabled = false, icon, value } = props;
 
   const appearance = useContextSelector(TabListContext, ctx => ctx.appearance);
+  const reserveSelectedTabSpace = useContextSelector(TabListContext, ctx => ctx.reserveSelectedTabSpace);
   const listDisabled = useContextSelector(TabListContext, ctx => ctx.disabled);
   const selected = useContextSelector(TabListContext, ctx => ctx.selectedValue === value);
   const onRegister = useContextSelector(TabListContext, ctx => ctx.onRegister);
@@ -64,6 +65,7 @@ export const useTab_unstable = (props: TabProps, ref: React.Ref<HTMLElement>): T
     iconOnly: Boolean(iconShorthand?.children && !contentShorthand.children),
     content: contentShorthand,
     appearance,
+    contentReservedSpaceClassName: reserveSelectedTabSpace ? '' : undefined,
     disabled,
     selected,
     size,

packages/react-components/react-tabs/src/components/Tab/useTabStyles.ts

@@ -12,6 +12,10 @@ export const tabClassNames: SlotClassNames<TabSlots> = {
   content: 'fui-Tab__content',
 };
 
+const reservedSpaceClassNames = {
+  content: 'fui-Tab__content--reserved-space',
+};
+
 /**
  * Styles for the root slot
  */
@@ -309,6 +313,8 @@ const useActiveIndicatorStyles = makeStyles({
  */
 const useIconStyles = makeStyles({
   base: {
+    gridColumnStart: 1,
+    gridRowStart: 1,
     alignItems: 'center',
     display: 'inline-flex',
     justifyContent: 'center',
@@ -341,6 +347,17 @@ const useContentStyles = makeStyles({
   selected: {
     ...typographyStyles.body1Strong,
   },
+  noIconBefore: {
+    gridColumnStart: 1,
+    gridRowStart: 1,
+  },
+  iconBefore: {
+    gridColumnStart: 2,
+    gridRowStart: 1,
+  },
+  placeholder: {
+    visibility: 'hidden',
+  },
 });
 
 /**
@@ -384,17 +401,31 @@ export const useTabStyles_unstable = (state: TabState): TabState => {
       size === 'small' &&
       (vertical ? activeIndicatorStyles.smallVertical : activeIndicatorStyles.smallHorizontal),
     selected && disabled && activeIndicatorStyles.disabled,
+
     state.root.className,
   );
 
   if (state.icon) {
     state.icon.className = mergeClasses(tabClassNames.icon, iconStyles.base, iconStyles[size], state.icon.className);
   }
 
+  // This needs to be before state.content.className is updated
+  if (state.contentReservedSpaceClassName !== undefined) {
+    state.contentReservedSpaceClassName = mergeClasses(
+      reservedSpaceClassNames.content,
+      contentStyles.base,
+      contentStyles.selected,
+      state.icon ? contentStyles.iconBefore : contentStyles.noIconBefore,
+      contentStyles.placeholder,
+      state.content.className,
+    );
+  }
+
   state.content.className = mergeClasses(
     tabClassNames.content,
     contentStyles.base,
     selected && contentStyles.selected,
+    state.icon ? contentStyles.iconBefore : contentStyles.noIconBefore,
     state.content.className,
   );
 

packages/react-components/react-tabs/src/components/TabList/TabList.types.ts

@@ -47,6 +47,15 @@ export type TabListProps = ComponentProps<TabListSlots> & {
    */
   appearance?: 'transparent' | 'subtle';
 
+  /**
+   * Tab size may change between unselected and selected states.
+   * The default scenario is a selected tab has bold text.
+   *
+   * When true, this property requests tabs be the same size whether unselected or selected.
+   * @default 'true'
+   */
+  reserveSelectedTabSpace?: boolean;
+
   /**
    * The value of the tab to be selected by default.
    * Typically useful when the selectedValue is uncontrolled.
@@ -82,7 +91,7 @@ export type TabListProps = ComponentProps<TabListSlots> & {
   vertical?: boolean;
 };
 
-export type TabListContextValue = Pick<TabListProps, 'onTabSelect' | 'selectedValue'> &
+export type TabListContextValue = Pick<TabListProps, 'onTabSelect' | 'selectedValue' | 'reserveSelectedTabSpace'> &
   Required<Pick<TabListProps, 'appearance' | 'disabled' | 'size' | 'vertical'>> & {
     /** A callback to allow a tab to register itself with the tab list. */
     onRegister: RegisterTabEventHandler;

packages/react-components/react-tabs/src/components/TabList/TabListContext.ts

@@ -5,6 +5,7 @@ import { TabListContextValue } from './TabList.types';
 // eslint-disable-next-line @fluentui/no-context-default-value
 export const TabListContext: Context<TabListContextValue> = createContext<TabListContextValue>({
   appearance: 'transparent',
+  reserveSelectedTabSpace: true,
   disabled: false,
   selectedValue: undefined,
   onRegister: () => {

packages/react-components/react-tabs/src/components/TabList/__snapshots__/TabList.test.tsx.snap

@@ -19,6 +19,11 @@ exports[`TabList renders tabs when disabled 1`] = `
       >
         First
       </span>
+      <span
+        class="fui-Tab__content--reserved-space"
+      >
+        First
+      </span>
     </button>
     <button
       class="fui-Tab"
@@ -45,6 +50,11 @@ exports[`TabList renders tabs when disabled 1`] = `
       >
         Third
       </span>
+      <span
+        class="fui-Tab__content--reserved-space"
+      >
+        Third
+      </span>
     </button>
   </div>
 </div>
@@ -70,6 +80,11 @@ exports[`TabList renders tabs with default selected tab 1`] = `
       >
         First
       </span>
+      <span
+        class="fui-Tab__content--reserved-space"
+      >
+        First
+      </span>
     </button>
     <button
       aria-selected="true"
@@ -98,6 +113,11 @@ exports[`TabList renders tabs with default selected tab 1`] = `
       >
         Third
       </span>
+      <span
+        class="fui-Tab__content--reserved-space"
+      >
+        Third
+      </span>
     </button>
   </div>
 </div>
@@ -133,6 +153,11 @@ exports[`TabList renders with tabs 1`] = `
       >
         First
       </span>
+      <span
+        class="fui-Tab__content--reserved-space"
+      >
+        First
+      </span>
     </button>
     <button
       aria-selected="false"
@@ -147,6 +172,11 @@ exports[`TabList renders with tabs 1`] = `
       >
         Second
       </span>
+      <span
+        class="fui-Tab__content--reserved-space"
+      >
+        Second
+      </span>
     </button>
     <button
       aria-selected="false"
@@ -161,6 +191,11 @@ exports[`TabList renders with tabs 1`] = `
       >
         Third
       </span>
+      <span
+        class="fui-Tab__content--reserved-space"
+      >
+        Third
+      </span>
     </button>
   </div>
 </div>

packages/react-components/react-tabs/src/components/TabList/useTabList.ts

@@ -19,7 +19,14 @@ import { TabValue } from '../Tab/Tab.types';
  * @param ref - reference to root HTMLElement of TabList
  */
 export const useTabList_unstable = (props: TabListProps, ref: React.Ref<HTMLElement>): TabListState => {
-  const { appearance = 'transparent', disabled = false, onTabSelect, size = 'medium', vertical = false } = props;
+  const {
+    appearance = 'transparent',
+    reserveSelectedTabSpace = true,
+    disabled = false,
+    onTabSelect,
+    size = 'medium',
+    vertical = false,
+  } = props;
 
   const innerRef = React.useRef<HTMLElement>(null);
 
@@ -81,6 +88,7 @@ export const useTabList_unstable = (props: TabListProps, ref: React.Ref<HTMLElem
       ...props,
     }),
     appearance,
+    reserveSelectedTabSpace,
     disabled,
     selectedValue,
     size,

packages/react-components/react-tabs/src/components/TabList/useTabListContextValues.tsx

@@ -3,6 +3,7 @@ import { TabListContextValue, TabListContextValues, TabListState } from './TabLi
 export function useTabListContextValues(state: TabListState): TabListContextValues {
   const {
     appearance,
+    reserveSelectedTabSpace,
     disabled,
     selectedValue: selectedKey,
     onRegister,
@@ -15,6 +16,7 @@ export function useTabListContextValues(state: TabListState): TabListContextValu
 
   const tabList: TabListContextValue = {
     appearance,
+    reserveSelectedTabSpace,
     disabled,
     selectedValue: selectedKey,
     onSelect,