From 9ebdb4f7f0c7e01f748a7fc3db4c515a8676bf44 Mon Sep 17 00:00:00 2001
From: Botond Ballo <botond@mozilla.com>
Date: Sun, 2 Apr 2017 17:20:23 -0400
Subject: [PATCH] Bug 1352564 - Add a method to invert a Matrix4x4 if it's
 invertible. r=botond

Use it in HitTestingTreeNode::Untransform.

MozReview-Commit-ID: 1JkLh99FD0h

--HG--
extra : rebase_source : ca9836c1999a2af3cac5287a9efa875c415634f7
---
 gfx/2d/Matrix.h                           | 10 ++++++++++
 gfx/layers/apz/src/HitTestingTreeNode.cpp |  6 +++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/gfx/2d/Matrix.h b/gfx/2d/Matrix.h
index 22a01ca1031c..6a3fcf4728e3 100644
--- a/gfx/2d/Matrix.h
+++ b/gfx/2d/Matrix.h
@@ -1236,6 +1236,16 @@ public:
     return clone;
   }
 
+  Maybe<Matrix4x4Typed<TargetUnits, SourceUnits>> MaybeInverse() const
+  {
+    typedef Matrix4x4Typed<TargetUnits, SourceUnits> InvertedMatrix;
+    InvertedMatrix clone = InvertedMatrix::FromUnknownMatrix(ToUnknownMatrix());
+    if (clone.Invert()) {
+      return Some(clone);
+    }
+    return Nothing();
+  }
+  
   void Normalize()
   {
       for (int i = 0; i < 4; i++) {
diff --git a/gfx/layers/apz/src/HitTestingTreeNode.cpp b/gfx/layers/apz/src/HitTestingTreeNode.cpp
index 8e916deeae18..0fe7ffa1d334 100644
--- a/gfx/layers/apz/src/HitTestingTreeNode.cpp
+++ b/gfx/layers/apz/src/HitTestingTreeNode.cpp
@@ -255,7 +255,11 @@ HitTestingTreeNode::Untransform(const ParentLayerPoint& aPoint) const
         mApzc
       ? mApzc->GetCurrentAsyncTransformWithOverscroll(AsyncPanZoomController::NORMAL)
       : AsyncTransformComponentMatrix());
-  return UntransformBy(transform.Inverse(), aPoint);
+  Maybe<ParentLayerToLayerMatrix4x4> inverse = transform.MaybeInverse();
+  if (inverse) {
+    return UntransformBy(inverse.ref(), aPoint);
+  }
+  return Nothing();
 }
 
 HitTestResult