[DevTools] Highlight the rect when the corresponding timeline bean is hovered (facebook#34881)

Stacked on facebook#34880.

In facebook#34861 I removed the highlight of the real view when hovering the
timeline since it was disruptive to stepping through the visuals.

This makes it so that when we hover the timeline we highlight the rect
with the subtle hover effect added in facebook#34880.

We can now just use the one shared state for this and don't need the CSS
psuedo-selectors.

<img width="603" height="813" alt="Screenshot 2025-10-16 at 3 11 17 PM"
src="https://github.com/user-attachments/assets/a018b5ce-dd4d-4e77-ad47-b4ea068f1976"
/>

Author: sebmarkbage

packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseRects.css

@@ -8,8 +8,8 @@
   background-color: color-mix(in srgb, var(--color-transition) 5%, transparent);
 }
 
-.SuspenseRectsContainer:hover:not(:has(.SuspenseRectsBoundary:hover))[data-highlighted='false'] {
-  outline-width: 1px;
+.SuspenseRectsContainer[data-hovered='true'] {
+  background-color: color-mix(in srgb, var(--color-transition) 15%, transparent);
 }
 
 .SuspenseRectsContainer[data-highlighted='true'] {
@@ -65,7 +65,7 @@
 }
 
 /* highlight this boundary */
-.SuspenseRectsBoundary:hover:not(:has(.SuspenseRectsBoundary:hover)) > .SuspenseRectsRect {
+.SuspenseRectsBoundary[data-hovered='true'] > .SuspenseRectsRect {
   background-color: color-mix(in srgb, var(--color-background) 50%, var(--color-suspense) 50%);
   transition: background-color 0.2s ease-out;
 }

packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseRects.js

@@ -37,6 +37,7 @@ function ScaledRect({
   visible,
   suspended,
   selected,
+  hovered,
   adjust,
   ...props
 }: {
@@ -45,6 +46,7 @@ function ScaledRect({
   visible: boolean,
   suspended: boolean,
   selected?: boolean,
+  hovered?: boolean,
   adjust?: boolean,
   ...
 }): React$Node {
@@ -61,6 +63,7 @@ function ScaledRect({
       data-visible={visible}
       data-suspended={suspended}
       data-selected={selected}
+      data-hovered={hovered}
       style={{
         // Shrink one pixel so that the bottom outline will line up with the top outline of the next one.
         width: adjust ? 'calc(' + width + ' - 1px)' : width,
@@ -80,7 +83,9 @@ function SuspenseRects({
   const store = useContext(StoreContext);
   const treeDispatch = useContext(TreeDispatcherContext);
   const suspenseTreeDispatch = useContext(SuspenseTreeDispatcherContext);
-  const {uniqueSuspendersOnly} = useContext(SuspenseTreeStateContext);
+  const {uniqueSuspendersOnly, timeline, hoveredTimelineIndex} = useContext(
+    SuspenseTreeStateContext,
+  );
 
   const {inspectedElementID} = useContext(TreeStateContext);
 
@@ -148,6 +153,9 @@ function SuspenseRects({
   // TODO: Use the nearest Suspense boundary
   const selected = inspectedElementID === suspenseID;
 
+  const hovered =
+    hoveredTimelineIndex > -1 && timeline[hoveredTimelineIndex] === suspenseID;
+
   const boundingBox = getBoundingBox(suspense.rects);
 
   return (
@@ -156,7 +164,8 @@ function SuspenseRects({
       className={styles.SuspenseRectsBoundary}
       visible={visible}
       selected={selected}
-      suspended={suspense.isSuspended}>
+      suspended={suspense.isSuspended}
+      hovered={hovered}>
       <ViewBox.Provider value={boundingBox}>
         {visible &&
           suspense.rects !== null &&
@@ -317,7 +326,7 @@ function SuspenseRectsContainer(): React$Node {
   const treeDispatch = useContext(TreeDispatcherContext);
   const suspenseTreeDispatch = useContext(SuspenseTreeDispatcherContext);
   // TODO: This relies on a full re-render of all children when the Suspense tree changes.
-  const {roots} = useContext(SuspenseTreeStateContext);
+  const {roots, hoveredTimelineIndex} = useContext(SuspenseTreeStateContext);
 
   // TODO: bbox does not consider uniqueSuspendersOnly filter
   const boundingBox = getDocumentBoundingRect(store, roots);
@@ -361,13 +370,15 @@ function SuspenseRectsContainer(): React$Node {
   }
 
   const isRootSelected = roots.includes(inspectedElementID);
+  const isRootHovered = hoveredTimelineIndex === 0;
 
   return (
     <div
       className={styles.SuspenseRectsContainer}
       onClick={handleClick}
       onDoubleClick={handleDoubleClick}
-      data-highlighted={isRootSelected}>
+      data-highlighted={isRootSelected}
+      data-hovered={isRootHovered}>
       <ViewBox.Provider value={boundingBox}>
         <div
           className={styles.SuspenseRectsViewBox}

packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseScrubber.css

@@ -54,8 +54,7 @@
   background: var(--color-transition);
 }
 
-.SuspenseScrubberStepHighlight > .SuspenseScrubberBead,
-.SuspenseScrubberStep:hover > .SuspenseScrubberBead {
+.SuspenseScrubberStepHighlight > .SuspenseScrubberBead {
   height: 0.75rem;
   transition: all 0.3s ease-out;
 }

packages/react-devtools-shared/src/devtools/views/SuspenseTab/SuspenseTimeline.js

@@ -53,10 +53,19 @@ function SuspenseTimelineInput() {
     switchSuspenseNode(timelineIndex);
   }
 
-  function handleHoverSegment(hoveredValue: number) {
-    // TODO: Consider highlighting the rect instead.
+  function handleHoverSegment(hoveredIndex: number) {
+    const nextSelectedSuspenseID = timeline[hoveredIndex];
+    suspenseTreeDispatch({
+      type: 'HOVER_TIMELINE_FOR_ID',
+      payload: nextSelectedSuspenseID,
+    });
+  }
+  function handleUnhoverSegment() {
+    suspenseTreeDispatch({
+      type: 'HOVER_TIMELINE_FOR_ID',
+      payload: -1,
+    });
   }
-  function handleUnhoverSegment() {}
 
   function skipPrevious() {
     const nextSelectedSuspenseID = timeline[timelineIndex - 1];