Bug 1700169 - Take into account the correct transform origin when scaling the thumb. r=tnikkel

The code was written assuming the transform origin is the top
of the scrollbar track, but it's actually the layer tree origin.

So far, scaling was only done for thumbs of the root scroll frame
where there is no difference, so no one noticed.

Depends on D114453

Differential Revision: https://phabricator.services.mozilla.com/D114454

gfx/layers/apz/src/ScrollThumbUtils.cpp

@@ -47,18 +47,22 @@ void AsyncScrollThumbTransformer::ScaleThumbBy(const Axis& aAxis,
                                                float aScale) {
   // To keep the position of the top of the thumb constant, the thumb needs to
   // translated to compensate for the scale applied. The origin with respect to
-  // which the scale is applied is the origin of the entire scrollbar, rather
-  // than the origin of the scroll thumb (meaning, for a vertical scrollbar it's
-  // at the top of the composition bounds). This means that empty space above
-  // the thumb is scaled too, effectively translating the thumb. We undo that
-  // translation here.
-  // (One can think of the adjustment being done to the translation here as
-  // a change of basis. We have a method to help with that,
-  // Matrix4x4::ChangeBasis(), but it wouldn't necessarily make the code
-  // cleaner in this case).
-  const CSSCoord thumbOrigin =
+  // which the scale is applied is the origin of the layer tree, rather than
+  // the origin of the scroll thumb. This means that the space between the
+  // origin and the top of thumb (including the part of the scrollbar track
+  // above the thumb, plus whatever content is above the scroll frame) is scaled
+  // too, effectively translating the thumb. We undo that translation here.
+  // (One can think of the adjustment being done to the translation here as a
+  // change of basis. We have a method to help with that,
+  // Matrix4x4::ChangeBasis(), but it wouldn't necessarily make the code cleaner
+  // in this case).
+  CSSCoord thumbOriginRelativeToCompBounds =
       (aAxis.GetPointOffset(mMetrics.GetVisualScrollOffset()) *
        mUnitlessThumbRatio);
+  CSSCoord compBoundsOrigin = aAxis.GetPointOffset(
+      mMetrics.CalculateCompositionBoundsInCssPixelsOfSurroundingContent()
+          .TopLeft());
+  CSSCoord thumbOrigin = compBoundsOrigin + thumbOriginRelativeToCompBounds;
   const CSSCoord thumbOriginScaled = thumbOrigin * aScale;
   const CSSCoord thumbOriginDelta = thumbOriginScaled - thumbOrigin;
   const ParentLayerCoord thumbOriginDeltaPL = thumbOriginDelta * mEffectiveZoom;