Improve DevTools Profiler commit-selector UX (facebook#20943)

* Improve DevTools Profiler commit-selector UX

1. Use natural log of durations (rather than linear) when calculating bar height. This reduces the impact of one (or few) outlier times on more common smaller durations. (Continue to use linear for bar color though.)
2. Decrease the minimum bar height to make the differences in height more noticeable.
3. Add a background hover highlight to increase contrast.
4. Add hover tooltip with commit duration and timestamp.

packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotCommitList.js

@@ -13,6 +13,8 @@ import AutoSizer from 'react-virtualized-auto-sizer';
 import {FixedSizeList} from 'react-window';
 import SnapshotCommitListItem from './SnapshotCommitListItem';
 import {minBarWidth} from './constants';
+import {formatDuration, formatTime} from './utils';
+import Tooltip from './Tooltip';
 
 import styles from './SnapshotCommitList.css';
 
@@ -24,6 +26,7 @@ export type ItemData = {|
   selectedCommitIndex: number | null,
   selectedFilteredCommitIndex: number | null,
   selectCommitIndex: (index: number) => void,
+  setHoveredCommitIndex: (index: number) => void,
   startCommitDrag: (newDragState: DragState) => void,
 |};
 
@@ -166,6 +169,10 @@ function List({
     }
   }, [dragState]);
 
+  const [hoveredCommitIndex, setHoveredCommitIndex] = useState<number | null>(
+    null,
+  );
+
   // Pass required contextual data down to the ListItem renderer.
   const itemData = useMemo<ItemData>(
     () => ({
@@ -176,6 +183,7 @@ function List({
       selectedCommitIndex,
       selectedFilteredCommitIndex,
       selectCommitIndex,
+      setHoveredCommitIndex,
       startCommitDrag: setDragState,
     }),
     [
@@ -186,22 +194,37 @@ function List({
       selectedCommitIndex,
       selectedFilteredCommitIndex,
       selectCommitIndex,
+      setHoveredCommitIndex,
     ],
   );
 
+  let tooltipLabel = null;
+  if (hoveredCommitIndex !== null) {
+    const commitDuration = commitDurations[hoveredCommitIndex];
+    const commitTime = commitTimes[hoveredCommitIndex];
+    tooltipLabel = `${formatDuration(commitDuration)}ms at ${formatTime(
+      commitTime,
+    )}s`;
+  }
+
   return (
-    <div ref={divRef} style={{height, width}}>
-      <FixedSizeList
-        className={styles.List}
-        layout="horizontal"
-        height={height}
-        itemCount={filteredCommitIndices.length}
-        itemData={itemData}
-        itemSize={itemSize}
-        ref={(listRef: any) /* Flow bug? */}
-        width={width}>
-        {SnapshotCommitListItem}
-      </FixedSizeList>
-    </div>
+    <Tooltip label={tooltipLabel}>
+      <div
+        ref={divRef}
+        style={{height, width}}
+        onMouseLeave={() => setHoveredCommitIndex(null)}>
+        <FixedSizeList
+          className={styles.List}
+          layout="horizontal"
+          height={height}
+          itemCount={filteredCommitIndices.length}
+          itemData={itemData}
+          itemSize={itemSize}
+          ref={(listRef: any) /* Flow bug? */}
+          width={width}>
+          {SnapshotCommitListItem}
+        </FixedSizeList>
+      </div>
+    </Tooltip>
   );
 }

packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotCommitListItem.css

@@ -7,10 +7,13 @@
   display: flex;
   align-items: flex-end;
 }
+.Outer:hover {
+  background-color: var(--color-background);
+}
 
 .Inner {
   width: 100%;
-  min-height: 5px;
+  min-height: 2px;
   background-color: var(--color-commit-did-not-render-fill);
   color: var(--color-commit-did-not-render-fill-text);
 }

packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotCommitListItem.js

@@ -31,6 +31,7 @@ function SnapshotCommitListItem({data: itemData, index, style}: Props) {
     maxDuration,
     selectedCommitIndex,
     selectCommitIndex,
+    setHoveredCommitIndex,
     startCommitDrag,
   } = itemData;
 
@@ -39,9 +40,21 @@ function SnapshotCommitListItem({data: itemData, index, style}: Props) {
   const commitDuration = commitDurations[index];
   const commitTime = commitTimes[index];
 
-  // Guard against commits with duration 0
-  const percentage =
+  // Use natural log for bar height.
+  // This prevents one (or a few) outliers from squishing the majority of other commits.
+  // So rather than e.g. _█_ we get something more like e.g. ▄█_
+  const heightScale =
+    Math.min(
+      1,
+      Math.max(0, Math.log(commitDuration) / Math.log(maxDuration)),
+    ) || 0;
+
+  // Use a linear scale for color.
+  // This gives some visual contrast between cheaper and more expensive commits
+  // and somewhat compensates for the log scale height.
+  const colorScale =
     Math.min(1, Math.max(0, commitDuration / maxDuration)) || 0;
+
   const isSelected = selectedCommitIndex === index;
 
   // Leave a 1px gap between snapshots
@@ -62,6 +75,7 @@ function SnapshotCommitListItem({data: itemData, index, style}: Props) {
     <div
       className={styles.Outer}
       onMouseDown={handleMouseDown}
+      onMouseEnter={() => setHoveredCommitIndex(index)}
       style={{
         ...style,
         width,
@@ -75,9 +89,9 @@ function SnapshotCommitListItem({data: itemData, index, style}: Props) {
       <div
         className={styles.Inner}
         style={{
-          height: `${Math.round(percentage * 100)}%`,
+          height: `${Math.round(heightScale * 100)}%`,
           backgroundColor:
-            percentage > 0 ? getGradientColor(percentage) : undefined,
+            commitDuration > 0 ? getGradientColor(colorScale) : undefined,
         }}
       />
     </div>

packages/react-devtools-shared/src/devtools/views/Profiler/SnapshotSelector.css

@@ -7,7 +7,7 @@
   height: 100%;
   min-width: 30px;
   margin-left: 0.25rem;
-  overflow: hidden;
+  overflow: visible;
 }
 .Commits:focus {
   outline: none;

packages/react-devtools-shared/src/devtools/views/Profiler/Tooltip.css

@@ -9,6 +9,7 @@
   background-color: var(--color-tooltip-background);
   color: var(--color-tooltip-text);
   opacity: 1;
+  white-space: nowrap;
   /* Make sure this is above the DevTools, which are above the Overlay */
   z-index: 10000002;
 }