Bug 1552608 - Handle fixed layer margins in ReceiveInputEvent. r=tnikkel

Only touch events are handled at this time.

Differential Revision: https://phabricator.services.mozilla.com/D48385

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

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -1790,6 +1790,7 @@ APZEventResult APZCTreeManager::ProcessTouchInput(MultiTouchInput& aInput) {
     HitTestResult hit = GetTouchInputBlockAPZC(aInput, &touchBehaviors);
     mApzcForInputBlock = hit.mTargetApzc;
     mHitResultForInputBlock = hit.mHitResult;
+    mFixedPosSidesForInputBlock = hit.mFixedPosSides;
     if (hit.mLayersId.IsValid()) {
       // Check for validity because we won't get a layers id for multi-touch
       // events.
@@ -1881,6 +1882,12 @@ APZEventResult APZCTreeManager::ProcessTouchInput(MultiTouchInput& aInput) {
           return result;
         }
         touchData.mScreenPoint = *untransformedScreenPoint;
+        if (mFixedPosSidesForInputBlock != eSideBitsNone) {
+          RecursiveMutexAutoLock lock(mTreeLock);
+          touchData.mScreenPoint +=
+              RoundedToInt(AsyncCompositionManager::ComputeFixedMarginsOffset(
+                  mFixedLayerMargins, mFixedPosSidesForInputBlock));
+        }
       }
     }
   }

gfx/layers/apz/src/APZCTreeManager.h

@@ -859,6 +859,13 @@ class APZCTreeManager : public IAPZCTreeManager, public APZInputBridge {
    * sync with mApzcForInputBlock.
    */
   gfx::CompositorHitTestInfo mHitResultForInputBlock;
+  /* If the current input event block is targeting an element that is fixed to
+   * the viewport, the sides of the viewport to which the element is fixed.
+   * Such elements may have been shifted to the dynamic toolbar, and this is
+   * used to offset event coordinates accordingly.
+   * This should be in sync with mApzcForInputBlock.
+   */
+  SideBits mFixedPosSidesForInputBlock;
   /* Sometimes we want to ignore all touches except one. In such cases, this
    * is set to the identifier of the touch we are not ignoring; in other cases,
    * this is set to -1.