Prevent QUnifiedTimer from ticking backwards.

This could happen in the following situation:
* a custom animation driver with fixed delta
* a triple-buffering scheme (rendering ahead a frame)
* a second animation timer starting while a first was active

This would cause QUnifiedTimer::startTimers() to trigger
QUnifiedTimer::updateAnimationTimers(-1), and use the current time from
the QElapsedTimer rather than the animation driver. This time could be
less than the last reported time from the animation driver.

Change-Id: Ibf1796fcb99f288d4946b30e5e7225695aa61781
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/src/corelib/animation/qabstractanimation.cpp b/src/corelib/animation/qabstractanimation.cpp
index 139876de3a..f7bb1e91bd 100644
--- a/src/corelib/animation/qabstractanimation.cpp
+++ b/src/corelib/animation/qabstractanimation.cpp
@@ -277,10 +277,12 @@ void QUnifiedTimer::updateAnimationTimers(qint64 currentTick)
 
     lastTick = totalElapsed;
 
-    //we make sure we only call update time if the time has actually changed
-    //it might happen in some cases that the time doesn't change because events are delayed
-    //when the CPU load is high
-    if (delta) {
+    //we make sure we only call update time if the time has actually advanced
+    //* it might happen in some cases that the time doesn't change because events are delayed
+    //  when the CPU load is high
+    //* it might happen in some cases that the delta is negative because the animation driver
+    //  advances faster than time.elapsed()
+    if (delta > 0) {
         insideTick = true;
         if (profilerCallback)
             profilerCallback(delta);