Bug 1556556 - Factor out an APZCCallbackHelper::GetCallbackTransform() helper. r=kats

This is to facilitate call sites that need to incorporate the transform into
a larger transform matrix rather than immediately applying the callback
transform to a point.

Differential Revision: https://phabricator.services.mozilla.com/D68275

gfx/layers/apz/util/APZCCallbackHelper.cpp

@@ -451,15 +451,14 @@ PresShell* APZCCallbackHelper::GetRootContentDocumentPresShellForContent(
   return context->PresShell();
 }
 
-CSSPoint APZCCallbackHelper::ApplyCallbackTransform(
-    const CSSPoint& aInput, const ScrollableLayerGuid& aGuid) {
-  CSSPoint input = aInput;
+mozilla::CSSToCSSMatrix4x4 APZCCallbackHelper::GetCallbackTransform(
+    const ScrollableLayerGuid& aGuid) {
   if (aGuid.mScrollId == ScrollableLayerGuid::NULL_SCROLL_ID) {
-    return input;
+    return {};
   }
   nsCOMPtr<nsIContent> content = nsLayoutUtils::FindContentFor(aGuid.mScrollId);
   if (!content) {
-    return input;
+    return {};
   }
 
   // First, scale inversely by the root content document's pres shell
@@ -469,16 +468,24 @@ CSSPoint APZCCallbackHelper::ApplyCallbackTransform(
   // compositor-side transforms) because Gecko needs it applied when hit
   // testing against content that's conceptually outside the resolution,
   // such as scrollbars.
+  float resolution = 1.0f;
   if (PresShell* presShell =
           GetRootContentDocumentPresShellForContent(content)) {
-    input = input / presShell->GetResolution();
+    resolution = presShell->GetResolution();
   }
 
   // Now apply the callback-transform. This is only approximately correct,
   // see the comment on GetCumulativeApzCallbackTransform for details.
   CSSPoint transform = nsLayoutUtils::GetCumulativeApzCallbackTransform(
       content->GetPrimaryFrame());
-  return input + transform;
+
+  return mozilla::CSSToCSSMatrix4x4::Scaling(1 / resolution, 1 / resolution, 1)
+      .PostTranslate(transform.x, transform.y, 0);
+}
+
+CSSPoint APZCCallbackHelper::ApplyCallbackTransform(
+    const CSSPoint& aInput, const ScrollableLayerGuid& aGuid) {
+  return GetCallbackTransform(aGuid).TransformPoint(aInput);
 }
 
 LayoutDeviceIntPoint APZCCallbackHelper::ApplyCallbackTransform(

gfx/layers/apz/util/APZCCallbackHelper.h

@@ -8,6 +8,7 @@
 
 #include "InputData.h"
 #include "LayersTypes.h"
+#include "Units.h"
 #include "mozilla/EventForwards.h"
 #include "mozilla/layers/APZUtils.h"
 #include "mozilla/layers/MatrixMessage.h"
@@ -97,9 +98,8 @@ class APZCCallbackHelper {
   static PresShell* GetRootContentDocumentPresShellForContent(
       nsIContent* aContent);
 
-  /* Apply a "callback transform" to |aInput|, which represents the coordinates
-     of an input event targeting content inside the scroll frame identified by
-    |aGuid|.
+  /* Return a "callback transform" to be applied to the coordinates of input
+     events targeting content inside the scroll frame identified by |aGuid|.
      The callback transform has two components:
        1. The pres shell resolution, representing the pinch-zoom scale
           (if the scroll frame |aScrollId| is inside the resolution, which
@@ -112,6 +112,10 @@ class APZCCallbackHelper {
          The translation is accumulated for all scroll frames form |aGuid| up to
          the root, using values populated in UpdateCallbackTransform. See that
          method's documentation for additional details. */
+  static CSSToCSSMatrix4x4 GetCallbackTransform(
+      const ScrollableLayerGuid& aGuid);
+
+  /* Apply |GetCallbackTransform()| to |aInput| and return the result. */
   static CSSPoint ApplyCallbackTransform(const CSSPoint& aInput,
                                          const ScrollableLayerGuid& aGuid);
 

layout/base/Units.h

@@ -224,6 +224,7 @@ typedef gfx::ScaleFactors2D<ParentLayerPixel, ScreenPixel>
 typedef gfx::ScaleFactors2D<ParentLayerPixel, ParentLayerPixel>
     ParentLayerToParentLayerScale2D;
 
+typedef gfx::Matrix4x4Typed<CSSPixel, CSSPixel> CSSToCSSMatrix4x4;
 typedef gfx::Matrix4x4Typed<LayoutDevicePixel, LayoutDevicePixel>
     LayoutDeviceToLayoutDeviceMatrix4x4;
 typedef gfx::Matrix4x4Typed<LayoutDevicePixel, ParentLayerPixel>