Fix CauseBacktrace type to allow stack to be null.

We want stack-less "causes" whenever we want to filter out a
stack completely, for example because it doesn't contain the
focused function when we're applying a focus function transform.

Pre-TypeScript, we were already using this field that way, even
though it wasn't typed correctly.

Author: mstange

src/components/shared/MarkerContextMenu.tsx

@@ -145,10 +145,10 @@ class MarkerContextMenuImpl extends PureComponent<Props> {
     return !marker || marker.end === null;
   }
 
-  _convertStackToString(stack: IndexIntoStackTable): string {
+  _convertStackToString(stack: IndexIntoStackTable | null): string {
     const { thread, implementationFilter } = this.props;
 
-    if (thread === null) {
+    if (thread === null || stack === null) {
       return '';
     }
 

src/components/tooltip/Marker.tsx

@@ -423,40 +423,43 @@ class MarkerTooltipContents extends React.PureComponent<Props> {
       categories,
     } = this.props;
     const { data, start } = marker;
-    if (data && 'cause' in data && data.cause) {
-      const { cause } = data;
-      const causeAge = cause.time !== undefined ? start - cause.time : 0;
-      return [
-        <TooltipDetailSeparator key="backtrace-separator" />,
-        <TooltipDetail label="Stack" key="backtrace">
-          <div className="tooltipDetailsBackTrace">
-            {
-              /* The cause's time might be later than the marker's start. For
+    if (!data || !('cause' in data) || !data.cause) {
+      return null;
+    }
+    const { time, stack } = data.cause;
+    if (stack === null) {
+      return null;
+    }
+    const causeAge = time !== undefined ? start - time : 0;
+    return [
+      <TooltipDetailSeparator key="backtrace-separator" />,
+      <TooltipDetail label="Stack" key="backtrace">
+        <div className="tooltipDetailsBackTrace">
+          {
+            /* The cause's time might be later than the marker's start. For
                 example this happens in some usual cases when the cause is
                 captured right when setting the end marker for tracing pairs of
                 markers. */
-              causeAge > 0 ? (
-                <h2 className="tooltipBackTraceTitle">
-                  {data.type === 'Styles' || marker.name === 'Reflow'
-                    ? `First invalidated ${formatTimestamp(
-                        causeAge
-                      )} before the flush, at:`
-                    : `Triggered ${formatTimestamp(causeAge)} ago, at:`}
-                </h2>
-              ) : null
-            }
-            <Backtrace
-              maxStacks={restrictHeightWidth ? 20 : Infinity}
-              stackIndex={cause.stack}
-              thread={thread}
-              implementationFilter={implementationFilter}
-              categories={categories}
-            />
-          </div>
-        </TooltipDetail>,
-      ];
-    }
-    return null;
+            causeAge > 0 ? (
+              <h2 className="tooltipBackTraceTitle">
+                {data.type === 'Styles' || marker.name === 'Reflow'
+                  ? `First invalidated ${formatTimestamp(
+                      causeAge
+                    )} before the flush, at:`
+                  : `Triggered ${formatTimestamp(causeAge)} ago, at:`}
+              </h2>
+            ) : null
+          }
+          <Backtrace
+            maxStacks={restrictHeightWidth ? 20 : Infinity}
+            stackIndex={stack}
+            thread={thread}
+            implementationFilter={implementationFilter}
+            categories={categories}
+          />
+        </div>
+      </TooltipDetail>,
+    ];
   }
 
   _maybeRenderNetworkPhases() {

src/profile-logic/merge-compare.ts

@@ -1356,7 +1356,8 @@ function mergeMarkers(
       if (oldData && 'cause' in oldData && oldData.cause) {
         // The old data has a cause, we need to convert the stack.
         const oldStack = oldData.cause.stack;
-        const newStack = translationMapForStacks.get(oldStack);
+        const newStack =
+          oldStack !== null ? translationMapForStacks.get(oldStack) : null;
         if (newStack === undefined) {
           throw new Error(
             `Missing old stack entry ${oldStack} in the translation map.`

src/types/markers.ts

@@ -201,7 +201,7 @@ export type CauseBacktrace = {
   // No upgrader was written for this change.
   tid?: Tid;
   time?: Milliseconds;
-  stack: IndexIntoStackTable;
+  stack: IndexIntoStackTable | null;
 };
 
 /**