Bug 1478776 - Part 7: Tune scroll events to only fire when the *relative* offset changes. r=botond

Internally, Gecko stores and updates the *absolute* offset between the visual
viewport and the page, however the spec demands that the scroll event be fired
whenever the *relative* offset between visual and layout viewport changes.

Differential Revision: https://phabricator.services.mozilla.com/D14044

--HG--
extra : moz-landing-system : lando

dom/base/VisualViewport.cpp

@@ -7,6 +7,7 @@
 #include "VisualViewport.h"
 
 #include "mozilla/EventDispatcher.h"
+#include "mozilla/ToString.h"
 #include "nsIScrollableFrame.h"
 #include "nsIDocShell.h"
 #include "nsPresContext.h"
@@ -175,23 +176,27 @@ void VisualViewport::FireResizeEvent() {
 
 /* ================= Scroll event handling ================= */
 
-void VisualViewport::PostScrollEvent() {
-  VVP_LOG("%p: PostScrollEvent\n", this);
+void VisualViewport::PostScrollEvent(const nsPoint& aPrevRelativeOffset) {
+  VVP_LOG("%p: PostScrollEvent, prevRelativeOffset %s\n", this,
+          ToString(aPrevRelativeOffset).c_str());
   if (mScrollEvent) {
     return;
   }
 
   // The event constructor will register itself with the refresh driver.
   if (nsPresContext* presContext = GetPresContext()) {
-    mScrollEvent = new VisualViewportScrollEvent(this, presContext);
+    mScrollEvent =
+        new VisualViewportScrollEvent(this, presContext, aPrevRelativeOffset);
     VVP_LOG("%p: PostScrollEvent, created new event\n", this);
   }
 }
 
 VisualViewport::VisualViewportScrollEvent::VisualViewportScrollEvent(
-    VisualViewport* aViewport, nsPresContext* aPresContext)
+    VisualViewport* aViewport, nsPresContext* aPresContext,
+    const nsPoint& aPrevRelativeOffset)
     : Runnable("VisualViewport::VisualViewportScrollEvent"),
-      mViewport(aViewport) {
+      mViewport(aViewport),
+      mPrevRelativeOffset(aPrevRelativeOffset) {
   aPresContext->RefreshDriver()->PostVisualViewportScrollEvent(this);
 }
 
@@ -205,12 +210,28 @@ VisualViewport::VisualViewportScrollEvent::Run() {
 
 void VisualViewport::FireScrollEvent() {
   MOZ_ASSERT(mScrollEvent);
+  nsPoint prevRelativeOffset = mScrollEvent->PrevRelativeOffset();
   mScrollEvent->Revoke();
   mScrollEvent = nullptr;
 
-  VVP_LOG("%p, FireScrollEvent, fire VisualViewport scroll\n", this);
-  WidgetGUIEvent event(true, eScroll, nullptr);
-  event.mFlags.mBubbles = false;
-  event.mFlags.mCancelable = false;
-  EventDispatcher::Dispatch(this, GetPresContext(), &event);
+  nsIPresShell* presShell = GetPresShell();
+  // Check whether the relative visual viewport offset actually changed - maybe
+  // both visual and layout viewport scrolled together and there was no change
+  // after all.
+  if (presShell) {
+    nsPoint curRelativeOffset =
+        presShell->GetVisualViewportOffsetRelativeToLayoutViewport();
+    VVP_LOG(
+        "%p: FireScrollEvent, curRelativeOffset %s, "
+        "prevRelativeOffset %s\n",
+        this, ToString(curRelativeOffset).c_str(),
+        ToString(prevRelativeOffset).c_str());
+    if (curRelativeOffset != prevRelativeOffset) {
+      VVP_LOG("%p, FireScrollEvent, fire VisualViewport scroll\n", this);
+      WidgetGUIEvent event(true, eScroll, nullptr);
+      event.mFlags.mBubbles = false;
+      event.mFlags.mCancelable = false;
+      EventDispatcher::Dispatch(this, GetPresContext(), &event);
+    }
+  }
 }

dom/base/VisualViewport.h

@@ -35,7 +35,7 @@ class VisualViewport final : public mozilla::DOMEventTargetHelper {
                                JS::Handle<JSObject*> aGivenProto) override;
 
   void PostResizeEvent();
-  void PostScrollEvent();
+  void PostScrollEvent(const nsPoint& aPrevRelativeOffset);
 
   // These two events are modelled after the ScrollEvent class in
   // nsGfxScrollFrame.h.
@@ -54,11 +54,23 @@ class VisualViewport final : public mozilla::DOMEventTargetHelper {
    public:
     NS_DECL_NSIRUNNABLE
     VisualViewportScrollEvent(VisualViewport* aViewport,
-                              nsPresContext* aPresContext);
+                              nsPresContext* aPresContext,
+                              const nsPoint& aPrevRelativeOffset);
     void Revoke() { mViewport = nullptr; }
+    nsPoint PrevRelativeOffset() const { return mPrevRelativeOffset; }
 
    private:
     VisualViewport* mViewport;
+    // The VisualViewport "scroll" event is supposed to be fired only when the
+    // *relative* offset between visual and layout viewport changes. The two
+    // viewports are updated independently from each other, though, so the only
+    // thing we can do is note the fact that one of the inputs into the relative
+    // visual viewport offset changed and then check the offset again at the
+    // next refresh driver tick, just before the event is going to fire.
+    // Hopefully, at this point both visual and layout viewport positions have
+    // been updated, so that we're able to tell whether the relative offset did
+    // in fact change or not.
+    const nsPoint mPrevRelativeOffset;
   };
 
  private:

gfx/layers/apz/test/mochitest/helper_basic_pan.html

@@ -55,7 +55,7 @@ function* test(testDriver) {
   // a constant zero and therefore not cause any visual viewport scroll events
   // to fire.
   visScrEvt.unregister();
-  todo_is(visScrEvt.count, 0, "Got no visual viewport scroll events");
+  is(visScrEvt.count, 0, "Got no visual viewport scroll events");
   visScrEvtInternal.unregister();
   // Our internal visual viewport scroll event on the other hand only cares
   // about the absolute offset of the visual viewport and should therefore

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -175,7 +175,8 @@ static ScreenMargin ScrollFrame(nsIContent* aContent,
     if (sf->IsRootScrollFrameOfDocument()) {
       if (nsCOMPtr<nsIPresShell> shell = GetPresShell(aContent)) {
         shell->SetVisualViewportOffset(
-            CSSPoint::ToAppUnits(aRequest.GetScrollOffset()));
+            CSSPoint::ToAppUnits(aRequest.GetScrollOffset()),
+            shell->GetVisualViewportOffsetRelativeToLayoutViewport());
       }
     }
   }

layout/base/PresShell.cpp

@@ -10042,12 +10042,13 @@ void nsIPresShell::SetVisualViewportSize(nscoord aWidth, nscoord aHeight) {
   }
 }
 
-void nsIPresShell::SetVisualViewportOffset(const nsPoint& aScrollOffset) {
+void nsIPresShell::SetVisualViewportOffset(const nsPoint& aScrollOffset,
+                                           const nsPoint& aPrevRelativeOffset) {
   if (mVisualViewportOffset != aScrollOffset) {
     mVisualViewportOffset = aScrollOffset;
 
     if (auto* window = nsGlobalWindowInner::Cast(mDocument->GetInnerWindow())) {
-      window->VisualViewport()->PostScrollEvent();
+      window->VisualViewport()->PostScrollEvent(aPrevRelativeOffset);
     }
   }
 }

layout/base/nsIPresShell.h

@@ -1647,7 +1647,8 @@ class nsIPresShell : public nsStubDocumentObserver {
     return mVisualViewportSize;
   }
 
-  void SetVisualViewportOffset(const nsPoint& aScrollOffset);
+  void SetVisualViewportOffset(const nsPoint& aScrollOffset,
+                               const nsPoint& aPrevRelativeOffset);
 
   nsPoint GetVisualViewportOffset() const { return mVisualViewportOffset; }
 

layout/generic/nsGfxScrollFrame.cpp

@@ -74,6 +74,7 @@
 #include "mozilla/layers/LayerTransactionChild.h"
 #include "mozilla/layers/ScrollLinkedEffectDetector.h"
 #include "mozilla/Unused.h"
+#include "VisualViewport.h"
 #include "LayersLogging.h"  // for Stringify
 #include <algorithm>
 #include <cstdlib>  // for std::abs(int/long)
@@ -2675,6 +2676,9 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
   if (dist.x >= horzAllowance || dist.y >= vertAllowance) {
     needFrameVisibilityUpdate = true;
   }
+  nsPoint prevVVRelativeOffset =
+      presContext->PresShell()
+          ->GetVisualViewportOffsetRelativeToLayoutViewport();
 
   // notify the listeners.
   for (uint32_t i = 0; i < mListeners.Length(); i++) {
@@ -2736,7 +2740,7 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
     // offset (e.g. by using nsIDOMWindowUtils.getVisualViewportOffset()
     // in chrome JS code) before it's updated by the next APZ repaint,
     // we could get incorrect results.
-    presContext->PresShell()->SetVisualViewportOffset(pt);
+    presContext->PresShell()->SetVisualViewportOffset(pt, prevVVRelativeOffset);
   }
 
   ScrollVisual();
@@ -2856,6 +2860,15 @@ void ScrollFrameHelper::ScrollToImpl(nsPoint aPt, const nsRect& aRange,
       nsPresContext::InteractionType::eScrollInteraction, TimeStamp::Now());
 
   PostScrollEvent();
+  // If this is a viewport scroll, this could affect the relative offset
+  // between layout and visual viewport, so we might have to fire a visual
+  // viewport scroll event as well.
+  if (mIsRoot) {
+    if (auto* window = nsGlobalWindowInner::Cast(
+            mOuter->PresContext()->Document()->GetInnerWindow())) {
+      window->VisualViewport()->PostScrollEvent(prevVVRelativeOffset);
+    }
+  }
 
   // notify the listeners.
   for (uint32_t i = 0; i < mListeners.Length(); i++) {