Reland "Do not coalesce wheel with phaseBegan to wheel with phaseEnded."

This reverts commit 5dd16566.

Reason for revert: This CL was confirmed to be the cause of the perf
regression in crbug.com/978980 - this is artificial so we can reland

Original change's description:
> Revert "Do not coalesce wheel with phaseBegan to wheel with phaseEnded."
>
> This reverts commit 2ca01526.
>
> Reason for revert: Speculative revert for https://crbug.com/978980
>
> Original change's description:
> > Do not coalesce wheel with phaseBegan to wheel with phaseEnded.
> >
> > If no element in the scrolling chain is not scrollable in the given direction
> > (delta_hints) on GSB we latch to the viewport. This is not user friendly on
> > platforms that do not have any scroll affordance animations (e.g. Windows)
> > since if the user changes the scrolling direction and an element in the chain
> > is actually scrollable in the new direction, the user expects it to scroll and
> > has no idea that we have latched to the (possibly unscrollable) viewport for
> > the rest of the scroll sequence. To address the issue for timer based wheel
> > scroll sequence we end the sequence and start a new one upon arrival of a wheel
> > event with different delta directions if no GSU has been consumed since the
> > beginning of the sequence. This cl ensures that the GSE/GSB generated for
> > breaking the latching seqeunce are not coalesced with each other.
> >
> > Bug: 882998
> > Change-Id: Icf703c935475fffd69b899c59abc03ec1b548784
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1524703
> > Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
> > Commit-Queue: Sahel Sharify <sahel@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#641719}
>
> TBR=nzolghadr@chromium.org,sahel@chromium.org
>
> # Not skipping CQ checks because original CL landed > 1 day ago.
>
> Bug: 882998,978980
> Change-Id: I85799b6e6eace029b4c2ef00e142f035ac9e8e4c
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1688230
> Reviewed-by: David Bokan <bokan@chromium.org>
> Commit-Queue: David Bokan <bokan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#674615}

TBR=bokan@chromium.org,nzolghadr@chromium.org,sahel@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 882998, 978980
Change-Id: I3dfb3f3ec0467b7101597720325222899e46f10b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716444Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680315}

ui/events/blink/blink_event_util.cc

@@ -317,17 +317,9 @@ bool HaveConsistentPhase(const WebMouseWheelEvent& event_to_coalesce,
   }
 
   if (event.has_synthetic_phase) {
-    // Synthetic phase information is added based on a timer in
-    // MouseWheelPhaseHandler. This information is for simulating scroll
-    // sequences when the beginning and end of scrolls are not available. It is
-    // alright to coalesce an event with synthetic phaseBegan to its previous
-    // event with synthetic phaseEnded since these phase values don't correspond
-    // with real start and end of the scroll sequences.
-    // It is also alright to coalesce a wheel event with synthetic phaseChanged
-    // to its previous one with synthetic phaseBegan.
-    return (event.phase == WebMouseWheelEvent::kPhaseEnded &&
-            event_to_coalesce.phase == WebMouseWheelEvent::kPhaseBegan) ||
-           (event.phase == WebMouseWheelEvent::kPhaseBegan &&
+    // It is alright to coalesce a wheel event with synthetic phaseChanged to
+    // its previous one with synthetic phaseBegan.
+    return (event.phase == WebMouseWheelEvent::kPhaseBegan &&
             event_to_coalesce.phase == WebMouseWheelEvent::kPhaseChanged);
   }
   return false;
@@ -377,18 +369,11 @@ void Coalesce(const WebMouseWheelEvent& event_to_coalesce,
       MergeDispatchTypes(old_dispatch_type, event_to_coalesce.dispatch_type);
   if (event_to_coalesce.has_synthetic_phase &&
       event_to_coalesce.phase != old_phase) {
-    if (event_to_coalesce.phase == WebMouseWheelEvent::kPhaseBegan) {
-      // Coalesce a wheel event with synthetic phase began with a wheel event
-      // with synthetic phase ended.
-      DCHECK_EQ(WebMouseWheelEvent::kPhaseEnded, old_phase);
-      event->phase = WebMouseWheelEvent::kPhaseChanged;
-    } else {
-      // Coalesce  a wheel event with synthetic phase changed to a wheel event
-      // with synthetic phase began.
-      DCHECK_EQ(WebMouseWheelEvent::kPhaseChanged, event_to_coalesce.phase);
-      DCHECK_EQ(WebMouseWheelEvent::kPhaseBegan, old_phase);
-      event->phase = WebMouseWheelEvent::kPhaseBegan;
-    }
+    // Coalesce  a wheel event with synthetic phase changed to a wheel event
+    // with synthetic phase began.
+    DCHECK_EQ(WebMouseWheelEvent::kPhaseChanged, event_to_coalesce.phase);
+    DCHECK_EQ(WebMouseWheelEvent::kPhaseBegan, old_phase);
+    event->phase = WebMouseWheelEvent::kPhaseBegan;
   }
 }
 

ui/events/blink/blink_event_util_unittest.cc

@@ -252,21 +252,24 @@ TEST(BlinkEventUtilTest, WebMouseWheelEventCoalescing) {
   coalesced_event.phase = blink::WebMouseWheelEvent::kPhaseEnded;
   EXPECT_FALSE(CanCoalesce(event_to_be_coalesced, coalesced_event));
 
+  // With timer based wheel scroll latching, we break the latching sequence on
+  // direction change when all prior GSU events in the current sequence are
+  // ignored. To do so we dispatch the pending wheel event with phaseEnded and
+  // the first wheel event in the opposite direction will have phaseBegan. The
+  // GSB generated from this wheel event will cause a new hittesting. To make
+  // sure that a GSB will actually get created we should not coalesce the wheel
+  // event with synthetic kPhaseBegan to one with synthetic kPhaseEnded.
   event_to_be_coalesced.has_synthetic_phase = true;
   coalesced_event.has_synthetic_phase = true;
-  EXPECT_TRUE(CanCoalesce(event_to_be_coalesced, coalesced_event));
-  Coalesce(event_to_be_coalesced, &coalesced_event);
-  EXPECT_EQ(blink::WebMouseWheelEvent::kPhaseChanged, coalesced_event.phase);
-  EXPECT_EQ(7, coalesced_event.delta_x);
-  EXPECT_EQ(9, coalesced_event.delta_y);
+  EXPECT_FALSE(CanCoalesce(event_to_be_coalesced, coalesced_event));
 
   event_to_be_coalesced.phase = blink::WebMouseWheelEvent::kPhaseChanged;
   coalesced_event.phase = blink::WebMouseWheelEvent::kPhaseBegan;
   EXPECT_TRUE(CanCoalesce(event_to_be_coalesced, coalesced_event));
   Coalesce(event_to_be_coalesced, &coalesced_event);
   EXPECT_EQ(blink::WebMouseWheelEvent::kPhaseBegan, coalesced_event.phase);
-  EXPECT_EQ(10, coalesced_event.delta_x);
-  EXPECT_EQ(13, coalesced_event.delta_y);
+  EXPECT_EQ(7, coalesced_event.delta_x);
+  EXPECT_EQ(9, coalesced_event.delta_y);
 
   event_to_be_coalesced.resending_plugin_id = 3;
   EXPECT_FALSE(CanCoalesce(event_to_be_coalesced, coalesced_event));