Bug 1705280 - Allow overscroll handoff to the root content APZC on pan gestures even if the root APZC is not scrollable to the given input directions. r=botond

To be precise the root APZC should actually be scrollable in the opposed
directions of the given input, it's checked in
AsyncPanZoomController::GetOverscrollableDirections.

Differential Revision: https://phabricator.services.mozilla.com/D113061

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -514,6 +514,9 @@ class AsyncPanZoomController {
   // overscroll-behavior).
   ScrollDirections GetAllowedHandoffDirections() const;
 
+  // Return the directions in which this APZC allows overscrolling.
+  ScrollDirections GetOverscrollableDirections() const;
+
   // Return whether or not a scroll delta will be able to scroll in either
   // direction.
   bool CanScroll(const ParentLayerPoint& aDelta) const;
@@ -1471,9 +1474,6 @@ class AsyncPanZoomController {
   void StartOverscrollAnimation(const ParentLayerPoint& aVelocity,
                                 SideBits aOverscrollSideBits);
 
-  // Return the directions in which this APZC allows overscrolling.
-  ScrollDirections GetOverscrollableDirections() const;
-
   // Start a smooth-scrolling animation to the given destination, with physics
   // based on the prefs for the indicated origin.
   void SmoothScrollTo(const CSSPoint& aDestination,

gfx/layers/apz/src/OverscrollHandoffState.cpp

@@ -152,6 +152,20 @@ RefPtr<AsyncPanZoomController> OverscrollHandoffChain::FindFirstScrollable(
       return mChain[i];
     }
 
+    // If there is any directions we allow overscroll effects on the root
+    // content APZC (i.e. the overscroll-behavior of the root one is not
+    // `none`), we consider the APZC can be scrollable in terms of pan gestures
+    // because it causes overscrolling even if it's not able to scroll to the
+    // direction.
+    if (StaticPrefs::apz_overscroll_enabled() &&
+        // FIXME: Bug 1707491: Drop this pan gesture input check.
+        aInput.mInputType == PANGESTURE_INPUT && mChain[i]->IsRootContent()) {
+      *aOutAllowedScrollDirections &= mChain[i]->GetOverscrollableDirections();
+      if (!aOutAllowedScrollDirections->isEmpty()) {
+        return mChain[i];
+      }
+    }
+
     *aOutAllowedScrollDirections &= mChain[i]->GetAllowedHandoffDirections();
     if (aOutAllowedScrollDirections->isEmpty()) {
       return nullptr;

gfx/layers/apz/test/gtest/TestOverscroll.cpp

@@ -5,6 +5,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "APZCBasicTester.h"
+#include "APZCTreeManagerTester.h"
 #include "APZTestCommon.h"
 #include "mozilla/layers/WebRenderScrollDataWrapper.h"
 
@@ -87,8 +88,7 @@ class APZCOverscrollTester : public APZCBasicTester {
     rootLayerScrollData.AppendScrollMetadata(scrollData, metadata);
     scrollData.AddLayerData(rootLayerScrollData);
 
-    registration =
-        MakeUnique<ScopedLayerTreeRegistration>(guid.mLayersId, mcc);
+    registration = MakeUnique<ScopedLayerTreeRegistration>(guid.mLayersId, mcc);
     tm->UpdateHitTestingTree(WebRenderScrollDataWrapper(*updater, &scrollData),
                              false, guid.mLayersId, 0);
     return guid;
@@ -1318,3 +1318,43 @@ TEST_F(APZCOverscrollTester, OverscrollByPanGesturesInterruptedByReflowZoom) {
   EXPECT_TRUE(!apzc->IsOverscrolled());
 }
 #endif
+
+class APZCOverscrollTesterForLayersOnly : public APZCTreeManagerTester {
+ public:
+  APZCOverscrollTesterForLayersOnly() { mLayersOnly = true; }
+
+  UniquePtr<ScopedLayerTreeRegistration> registration;
+  TestAsyncPanZoomController* rootApzc;
+};
+
+TEST_F(APZCOverscrollTesterForLayersOnly, OverscrollHandoff) {
+  SCOPED_GFX_PREF_BOOL("apz.overscroll.enabled", true);
+
+  const char* layerTreeSyntax = "c(c)";
+  nsIntRegion layerVisibleRegion[] = {nsIntRegion(IntRect(0, 0, 100, 100)),
+                                      nsIntRegion(IntRect(0, 0, 100, 50))};
+  root =
+      CreateLayerTree(layerTreeSyntax, layerVisibleRegion, nullptr, lm, layers);
+  SetScrollableFrameMetrics(root, ScrollableLayerGuid::START_SCROLL_ID,
+                            CSSRect(0, 0, 200, 200));
+  SetScrollableFrameMetrics(layers[1], ScrollableLayerGuid::START_SCROLL_ID + 1,
+                            // same size as the visible region so that
+                            // the container is not scrollable in any directions
+                            // actually. This is simulating overflow: hidden
+                            // iframe document in Fission, though we don't set
+                            // a different layers id.
+                            CSSRect(0, 0, 100, 50));
+
+  SetScrollHandoff(layers[1], root);
+
+  registration =
+      MakeUnique<ScopedLayerTreeRegistration>(LayersId{0}, root, mcc);
+  UpdateHitTestingTree();
+  rootApzc = ApzcOf(root);
+  rootApzc->GetFrameMetrics().SetIsRootContent(true);
+
+  // A pan gesture on the child scroller (which is not scrollable though).
+  PanGesture(PanGestureInput::PANGESTURE_START, manager, ScreenIntPoint(50, 20),
+             ScreenPoint(0, -2), mcc->Time());
+  EXPECT_TRUE(rootApzc->IsOverscrolled());
+}