Bug 528208. Measure interval delays from start of callback to start of callback, not from end of callback to start of callback. r=peterv

dom/base/nsGlobalWindow.cpp

@@ -8099,14 +8099,21 @@ nsGlobalWindow::RunTimeout(nsTimeout *aTimeout)
     // If we have a regular interval timer, we re-schedule the
     // timeout, accounting for clock drift.
     if (timeout->mInterval) {
-      // Compute time to next timeout for interval timer. If we're
-      // running pending timeouts because they've been temporarily
-      // disabled (!aTimeout), set the next interval to be relative to
-      // "now", and not to when the timeout that was pending should
-      // have fired.
-      // Also check if the next interval timeout is overdue.  If so,
-      // then restart the interval from now.
+      // Compute time to next timeout for interval timer.
       PRTime nextInterval = (PRTime)timeout->mInterval * PR_USEC_PER_MSEC;
+
+      // Make sure nextInterval is at least DOM_MIN_TIMEOUT_VALUE.
+      // Note: We must cast the rhs expression to PRTime to work
+      // around what looks like a compiler bug on x86_64.
+      if (nextInterval < (PRTime)(DOM_MIN_TIMEOUT_VALUE * PR_USEC_PER_MSEC)) {
+         nextInterval = DOM_MIN_TIMEOUT_VALUE * PR_USEC_PER_MSEC;
+      }
+
+      // If we're running pending timeouts because they've been temporarily
+      // disabled (!aTimeout), set the next interval to be relative to "now",
+      // and not to when the timeout that was pending should have fired.  Also
+      // check if the next interval timeout is overdue.  If so, then restart
+      // the interval from now.
       if (!aTimeout || nextInterval + timeout->mWhen <= now)
         nextInterval += now;
       else
@@ -8114,11 +8121,10 @@ nsGlobalWindow::RunTimeout(nsTimeout *aTimeout)
 
       PRTime delay = nextInterval - PR_Now();
 
-      // Make sure the delay is at least DOM_MIN_TIMEOUT_VALUE.
-      // Note: We must cast the rhs expression to PRTime to work
-      // around what looks like a compiler bug on x86_64.
-      if (delay < (PRTime)(DOM_MIN_TIMEOUT_VALUE * PR_USEC_PER_MSEC)) {
-        delay = DOM_MIN_TIMEOUT_VALUE * PR_USEC_PER_MSEC;
+      // And make sure delay is nonnegative; that might happen if the timer
+      // thread is firing our timers somewhat early.
+      if (delay < 0) {
+        delay = 0;
       }
 
       if (timeout->mTimer) {