Bug 1509575 - Extend the internal visual scroll API to allow specifying "restore" vs. regular priority. r=kats

The distinction is not exposed at the JS level which currently always uses
"restore", but it could be if necessary.

Differential Revision: https://phabricator.services.mozilla.com/D16346

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

dom/base/nsDOMWindowUtils.cpp

@@ -1406,8 +1406,12 @@ nsDOMWindowUtils::ScrollToVisual(float aOffsetX, float aOffsetY) {
   // This should only be called on the root content document.
   NS_ENSURE_TRUE(presContext->IsRootContentDocument(), NS_ERROR_INVALID_ARG);
 
-  presContext->PresShell()->SetPendingVisualViewportOffset(
-      Some(CSSPoint::ToAppUnits(CSSPoint(aOffsetX, aOffsetY))));
+  // Use |eRestore| as the priority for now, as it's the conservative choice.
+  // If a JS call site needs higher priority, we can expose the update type
+  // as a parameter.
+  presContext->PresShell()->SetPendingVisualScrollUpdate(
+      CSSPoint::ToAppUnits(CSSPoint(aOffsetX, aOffsetY)),
+      FrameMetrics::eRestore);
 
   return NS_OK;
 }

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -4262,8 +4262,7 @@ void AsyncPanZoomController::NotifyLayersUpdated(
       (aLayerMetrics.GetScrollGeneration() != Metrics().GetScrollGeneration());
 
   if (scrollOffsetUpdated && userScrolled &&
-      aLayerMetrics.GetScrollUpdateType() ==
-          FrameMetrics::ScrollOffsetUpdateType::eRestore) {
+      aLayerMetrics.GetScrollUpdateType() == FrameMetrics::eRestore) {
     APZC_LOG(
         "%p dropping scroll update of type eRestore because of user scroll\n",
         this);
@@ -4274,6 +4273,23 @@ void AsyncPanZoomController::NotifyLayersUpdated(
       aLayerMetrics.GetDoSmoothScroll() &&
       (aLayerMetrics.GetScrollGeneration() != Metrics().GetScrollGeneration());
 
+  // The main thread may also ask us to scroll the visual viewport to a
+  // particular location. This is different from a layout viewport offset update
+  // in that the layout viewport offset is limited to the layout scroll range
+  // (this will be enforced by the main thread once bug 1516056 is fixed),
+  // while the visual viewport offset is not.
+  // The update type indicates the priority; an eMainThread layout update (or
+  // a smooth scroll request which is similar) takes precedence over an eRestore
+  // visual update, but we allow the possibility for the main thread to ask us
+  // to scroll both the layout and visual viewports to distinct (but compatible)
+  // locations (via e.g. both updates being eRestore).
+  bool visualScrollOffsetUpdated =
+      aLayerMetrics.GetVisualScrollUpdateType() != FrameMetrics::eNone;
+  if (aLayerMetrics.GetScrollUpdateType() == FrameMetrics::eMainThread ||
+      smoothScrollRequested) {
+    visualScrollOffsetUpdated = false;
+  }
+
   // TODO if we're in a drag and scrollOffsetUpdated is set then we want to
   // ignore it
 
@@ -4500,18 +4516,7 @@ void AsyncPanZoomController::NotifyLayersUpdated(
     SmoothScrollTo(Metrics().GetSmoothScrollOffset());
   }
 
-  // If the main thread asked us to scroll the visual viewport to a particular
-  // location, do so. This is different from a layout viewport offset update
-  // in that the layout viewport offset is limited to the layout scroll range
-  // (this will be enforced by the main thread once bug 1516056 is fixed),
-  // while the visual viewport offset is not. The main thread can also ask
-  // us to scroll both the layout and visual viewports to distinct (but
-  // compatible) locations.
-  FrameMetrics::ScrollOffsetUpdateType visualUpdateType =
-      aLayerMetrics.GetVisualScrollUpdateType();
-  MOZ_ASSERT(visualUpdateType == FrameMetrics::eNone ||
-             visualUpdateType == FrameMetrics::eMainThread);
-  if (visualUpdateType == FrameMetrics::eMainThread) {
+  if (visualScrollOffsetUpdated) {
     Metrics().ClampAndSetScrollOffset(aLayerMetrics.GetVisualViewportOffset());
 
     // The rest of this branch largely follows the code in the

layout/base/nsIPresShell.h

@@ -18,6 +18,7 @@
 #include "mozilla/StyleSheet.h"
 #include "mozilla/UniquePtr.h"
 #include "mozilla/WeakPtr.h"
+#include "FrameMetrics.h"
 #include "GeckoProfiler.h"
 #include "gfxPoint.h"
 #include "nsTHashtable.h"
@@ -176,6 +177,7 @@ class nsIPresShell : public nsStubDocumentObserver {
 
  protected:
   typedef mozilla::dom::Document Document;
+  typedef mozilla::layers::FrameMetrics FrameMetrics;
   typedef mozilla::layers::LayerManager LayerManager;
   typedef mozilla::gfx::SourceSurface SourceSurface;
 
@@ -1660,18 +1662,32 @@ class nsIPresShell : public nsStubDocumentObserver {
 
   nsPoint GetVisualViewportOffsetRelativeToLayoutViewport() const;
 
-  // Ask APZ in the next transaction to scroll to the given visual viewport 
+  // Represents an update to the visual scroll offset that will be sent to APZ.
+  // The update type is used to determine priority compared to other scroll
+  // updates.
+  struct VisualScrollUpdate {
+    nsPoint mVisualScrollOffset;
+    FrameMetrics::ScrollOffsetUpdateType mUpdateType;
+  };
+
+  // Ask APZ in the next transaction to scroll to the given visual viewport
   // offset (relative to the document).
   // Use this sparingly, as it will clobber JS-driven scrolling that happens
   // in the same frame. This is mostly intended to be used in special
   // situations like "first paint" or session restore.
   // Please request APZ review if adding a new call site.
-  void SetPendingVisualViewportOffset(
-      const mozilla::Maybe<nsPoint>& aPendingVisualViewportOffset) {
-    mPendingVisualViewportOffset = aPendingVisualViewportOffset;
+  void SetPendingVisualScrollUpdate(
+      const nsPoint& aVisualViewportOffset,
+      FrameMetrics::ScrollOffsetUpdateType aUpdateType) {
+    mPendingVisualScrollUpdate =
+        mozilla::Some(VisualScrollUpdate{aVisualViewportOffset, aUpdateType});
   }
-  const mozilla::Maybe<nsPoint>& GetPendingVisualViewportOffset() const {
-    return mPendingVisualViewportOffset;
+  void ClearPendingVisualScrollUpdate() {
+    mPendingVisualScrollUpdate = mozilla::Nothing();
+  }
+  const mozilla::Maybe<VisualScrollUpdate>& GetPendingVisualScrollUpdate()
+      const {
+    return mPendingVisualScrollUpdate;
   }
 
   nsPoint GetLayoutViewportOffset() const;
@@ -1755,10 +1771,10 @@ class nsIPresShell : public nsStubDocumentObserver {
 
   nsPoint mVisualViewportOffset;
 
-  // A pending visual viewport offset that we will ask APZ to scroll to
+  // A pending visual scroll offset that we will ask APZ to scroll to
   // during the next transaction. Cleared when we send the transaction.
   // Only applicable to the RCD pres shell.
-  mozilla::Maybe<nsPoint> mPendingVisualViewportOffset;
+  mozilla::Maybe<VisualScrollUpdate> mPendingVisualScrollUpdate;
 
   // A list of stack weak frames. This is a pointer to the last item in the
   // list.

layout/base/nsLayoutUtils.cpp

@@ -8708,11 +8708,12 @@ static void MaybeReflowForInflationScreenSizeChange(
     metrics.SetBaseScrollOffset(apzScrollPosition);
 
     if (aIsRootContent) {
-      if (const Maybe<nsPoint>& visualOffset =
-              presShell->GetPendingVisualViewportOffset()) {
-        metrics.SetVisualViewportOffset(CSSPoint::FromAppUnits(*visualOffset));
-        metrics.SetVisualScrollUpdateType(FrameMetrics::eMainThread);
-        presShell->SetPendingVisualViewportOffset(Nothing());
+      if (const Maybe<nsIPresShell::VisualScrollUpdate>& visualUpdate =
+              presShell->GetPendingVisualScrollUpdate()) {
+        metrics.SetVisualViewportOffset(
+            CSSPoint::FromAppUnits(visualUpdate->mVisualScrollOffset));
+        metrics.SetVisualScrollUpdateType(visualUpdate->mUpdateType);
+        presShell->ClearPendingVisualScrollUpdate();
       }
     }