Fix priority inference of next level of work (#15478)

Bugfix for `inferPriorityFromExpirationTime` function. It happened to
work in our existing tests because we use virtual time.

Flow would have caught this if expiration times were an opaque type. We
should consider that in the future. (The downside of opaque types is
that all operations would have to go through helper functions, which may
or may not get inlined by Closure.)

packages/react-reconciler/src/ReactFiberExpirationTime.js

@@ -111,14 +111,14 @@ export function inferPriorityFromExpirationTime(
     return IdlePriority;
   }
   const msUntil =
-    msToExpirationTime(expirationTime) - msToExpirationTime(currentTime);
+    expirationTimeToMs(expirationTime) - expirationTimeToMs(currentTime);
   if (msUntil <= 0) {
     return ImmediatePriority;
   }
-  if (msUntil <= HIGH_PRIORITY_EXPIRATION) {
+  if (msUntil <= HIGH_PRIORITY_EXPIRATION + HIGH_PRIORITY_BATCH_SIZE) {
     return UserBlockingPriority;
   }
-  if (msUntil <= LOW_PRIORITY_EXPIRATION) {
+  if (msUntil <= LOW_PRIORITY_EXPIRATION + LOW_PRIORITY_BATCH_SIZE) {
     return NormalPriority;
   }
 

packages/react-reconciler/src/__tests__/ReactSchedulerIntegration-test.internal.js

@@ -150,6 +150,22 @@ describe('ReactSchedulerIntegration', () => {
     ]);
   });
 
+  it('after completing a level of work, infers priority of the next batch based on its expiration time', () => {
+    function App({label}) {
+      Scheduler.yieldValue(`${label} [${getCurrentPriorityAsString()}]`);
+      return label;
+    }
+
+    // Schedule two separate updates at different priorities
+    runWithPriority(UserBlockingPriority, () => {
+      ReactNoop.render(<App label="A" />);
+    });
+    ReactNoop.render(<App label="B" />);
+
+    // The second update should run at normal priority
+    expect(Scheduler).toFlushAndYield(['A [UserBlocking]', 'B [Normal]']);
+  });
+
   // TODO
   it.skip('passive effects have render priority even if they are flushed early', () => {});
 });