Bug 1590582 - Subtract the fixed margins offset when adjusting event coordinates during hit testing, rather than adding it. r=tnikkel

The offset is applied (added) during rendering; it needs to be un-applied
(subtracted) during hit testing.

Depends on D50360

Differential Revision: https://phabricator.services.mozilla.com/D50361

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

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -1881,7 +1881,7 @@ APZEventResult APZCTreeManager::ProcessTouchInput(MultiTouchInput& aInput) {
         touchData.mScreenPoint = *untransformedScreenPoint;
         if (mFixedPosSidesForInputBlock != eSideBitsNone) {
           RecursiveMutexAutoLock lock(mTreeLock);
-          touchData.mScreenPoint +=
+          touchData.mScreenPoint -=
               RoundedToInt(AsyncCompositionManager::ComputeFixedMarginsOffset(
                   mFixedLayerMargins, mFixedPosSidesForInputBlock));
         }