Bug 1787974 - Fixed small logic change in FindNextFireTimeForCurrentThread(). r=smaug

I also took a stab at documenting the behavior since I found it a bit
difficult to follow.

Differential Revision: https://phabricator.services.mozilla.com/D165120

xpcom/threads/TimerThread.cpp

@@ -779,12 +779,12 @@ TimeStamp TimerThread::FindNextFireTimeForCurrentThread(TimeStamp aDefault,
   AUTO_TIMERS_STATS(TimerThread_FindNextFireTimeForCurrentThread);
 
   for (const Entry& entry : mTimers) {
-    if (entry.Timeout() >= aDefault) {
-      return aDefault;
-    }
-
     const nsTimerImpl* timer = entry.Value();
     if (timer) {
+      if (entry.Timeout() > aDefault) {
+        return aDefault;
+      }
+
       // Don't yield to timers created with the *_LOW_PRIORITY type.
       if (!timer->IsLowPriority()) {
         bool isOnCurrentThread = false;

xpcom/threads/TimerThread.h

@@ -47,6 +47,10 @@ class TimerThread final : public mozilla::Runnable, public nsIObserver {
       MOZ_REQUIRES(aTimer->mMutex);
   nsresult RemoveTimer(nsTimerImpl* aTimer, const MutexAutoLock& aProofOfLock)
       MOZ_REQUIRES(aTimer->mMutex);
+  // Considering only the first 'aSearchBound' timers (in firing order), returns
+  // the timeout of the first non-low-priority timer, on the current thread,
+  // that will fire before 'aDefault'. If no such timer exists, 'aDefault' is
+  // returned.
   TimeStamp FindNextFireTimeForCurrentThread(TimeStamp aDefault,
                                              uint32_t aSearchBound);