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diff --git a/gfx/layers/apz/src/ScrollThumbUtils.cpp b/gfx/layers/apz/src/ScrollThumbUtils.cpp
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+++ b/gfx/layers/apz/src/ScrollThumbUtils.cpp
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "ScrollThumbUtils.h"
+#include "AsyncPanZoomController.h"
+#include "FrameMetrics.h"
+#include "UnitTransforms.h"
+#include "Units.h"
+#include "gfxPlatform.h"
+#include "mozilla/gfx/Matrix.h"
+#include "mozilla/StaticPrefs_toolkit.h"
+
+namespace mozilla {
+namespace layers {
+namespace apz {
+
+struct AsyncScrollThumbTransformer {
+  // Inputs
+  const LayerToParentLayerMatrix4x4& mCurrentTransform;
+  const gfx::Matrix4x4& mScrollableContentTransform;
+  AsyncPanZoomController* mApzc;
+  const FrameMetrics& mMetrics;
+  const ScrollbarData& mScrollbarData;
+  bool mScrollbarIsDescendant;
+
+  // Intermediate results
+  AsyncTransformComponentMatrix mAsyncTransform;
+  AsyncTransformComponentMatrix mScrollbarTransform;
+
+  LayerToParentLayerMatrix4x4 ComputeTransform();
+
+ private:
+  // Helper functions for ComputeTransform().
+
+  // If the thumb's orientation is along |aAxis|, add transformations
+  // of the thumb into |mScrollbarTransform|.
+  void ApplyTransformForAxis(const Axis& aAxis);
+
+  enum class ScrollThumbExtent { Start, End };
+
+  // Scale the thumb by |aScale| along |aAxis|, while keeping constant the
+  // position of the top denoted by |aExtent|.
+  void ScaleThumbBy(const Axis& aAxis, float aScale, ScrollThumbExtent aExtent);
+
+  // Translate the thumb along |aAxis| by |aTranslation| in "scrollbar space"
+  // (CSS pixels along the scrollbar track, similar to e.g.
+  // |mScrollbarData.mThumbStart|).
+  void TranslateThumb(const Axis& aAxis, OuterCSSCoord aTranslation);
+};
+
+void AsyncScrollThumbTransformer::TranslateThumb(const Axis& aAxis,
+                                                 OuterCSSCoord aTranslation) {
+  aAxis.PostTranslate(
+      mScrollbarTransform,
+      ViewAs<CSSPixel>(aTranslation,
+                       PixelCastJustification::CSSPixelsOfSurroundingContent) *
+          mMetrics.GetDevPixelsPerCSSPixel() *
+          LayoutDeviceToParentLayerScale(1.0));
+}
+
+void AsyncScrollThumbTransformer::ScaleThumbBy(const Axis& aAxis, float aScale,
+                                               ScrollThumbExtent aExtent) {
+  // 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 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, the part of the scrollbar above the track (i.e. a
+  // scrollbar button if present), 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).
+  const OuterCSSCoord scrollTrackOrigin =
+      aAxis.GetPointOffset(
+          mMetrics.CalculateCompositionBoundsInOuterCssPixels().TopLeft()) +
+      mScrollbarData.mScrollTrackStart;
+  OuterCSSCoord thumbExtent = scrollTrackOrigin + mScrollbarData.mThumbStart;
+  if (aExtent == ScrollThumbExtent::End) {
+    thumbExtent += mScrollbarData.mThumbLength;
+  }
+  const OuterCSSCoord thumbExtentScaled = thumbExtent * aScale;
+  const OuterCSSCoord thumbExtentDelta = thumbExtentScaled - thumbExtent;
+
+  aAxis.PostScale(mScrollbarTransform, aScale);
+  TranslateThumb(aAxis, -thumbExtentDelta);
+}
+
+void AsyncScrollThumbTransformer::ApplyTransformForAxis(const Axis& aAxis) {
+  ParentLayerCoord asyncScroll = aAxis.GetTransformTranslation(mAsyncTransform);
+  const float asyncZoom = aAxis.GetTransformScale(mAsyncTransform);
+  const ParentLayerCoord overscroll =
+      aAxis.GetPointOffset(mApzc->GetOverscrollAmount());
+
+  bool haveAsyncZoom = !FuzzyEqualsAdditive(asyncZoom, 1.f);
+  if (!haveAsyncZoom && mApzc->IsZero(asyncScroll) &&
+      mApzc->IsZero(overscroll)) {
+    return;
+  }
+
+  OuterCSSCoord translation;
+  float scale = 1.0;
+
+  bool recalcMode = StaticPrefs::apz_scrollthumb_recalc();
+  if (recalcMode) {
+    // In this branch (taken when apz.scrollthumb.recalc=true), |translation|
+    // and |scale| are computed using the approach implemented in bug 1554795
+    // of fully recalculating the desired position and size using the logic
+    // that attempts to closely match the main-thread calculation.
+
+    const CSSRect visualViewportRect = mApzc->GetCurrentAsyncVisualViewport(
+        AsyncPanZoomController::eForCompositing);
+    const CSSCoord visualViewportLength =
+        aAxis.GetRectLength(visualViewportRect);
+
+    const CSSCoord maxMinPosDifference =
+        CSSCoord(
+            aAxis.GetRectLength(mMetrics.GetScrollableRect()).Truncated()) -
+        visualViewportLength;
+
+    OuterCSSCoord effectiveThumbLength = mScrollbarData.mThumbLength;
+
+    if (haveAsyncZoom) {
+      // The calculations here closely follow the main thread calculations at
+      // https://searchfox.org/mozilla-central/rev/0bf957f909ae1f3d19b43fd4edfc277342554836/layout/generic/nsGfxScrollFrame.cpp#6902-6927
+      // and
+      // https://searchfox.org/mozilla-central/rev/0bf957f909ae1f3d19b43fd4edfc277342554836/layout/xul/nsSliderFrame.cpp#587-614
+      // Any modifications there should be reflected here as well.
+      const CSSCoord pageIncrementMin =
+          static_cast<int>(visualViewportLength * 0.8);
+      CSSCoord pageIncrement;
+
+      CSSToLayoutDeviceScale deviceScale = mMetrics.GetDevPixelsPerCSSPixel();
+      if (*mScrollbarData.mDirection == ScrollDirection::eVertical) {
+        const CSSCoord lineScrollAmount =
+            (mApzc->GetScrollMetadata().GetLineScrollAmount() / deviceScale)
+                .height;
+        const double kScrollMultiplier =
+            StaticPrefs::toolkit_scrollbox_verticalScrollDistance();
+        CSSCoord increment = lineScrollAmount * kScrollMultiplier;
+
+        pageIncrement =
+            std::max(visualViewportLength - increment, pageIncrementMin);
+      } else {
+        pageIncrement = pageIncrementMin;
+      }
+
+      float ratio = pageIncrement / (maxMinPosDifference + pageIncrement);
+
+      OuterCSSCoord desiredThumbLength{
+          std::max(mScrollbarData.mThumbMinLength,
+                   mScrollbarData.mScrollTrackLength * ratio)};
+
+      // Round the thumb length to an integer number of LayoutDevice pixels, to
+      // match the main-thread behaviour.
+      auto outerDeviceScale = ViewAs<OuterCSSToLayoutDeviceScale>(
+          deviceScale, PixelCastJustification::CSSPixelsOfSurroundingContent);
+      desiredThumbLength =
+          LayoutDeviceCoord((desiredThumbLength * outerDeviceScale).Rounded()) /
+          outerDeviceScale;
+
+      effectiveThumbLength = desiredThumbLength;
+
+      scale = desiredThumbLength / mScrollbarData.mThumbLength;
+    }
+
+    // Subtracting the offset of the scrollable rect is needed for right-to-left
+    // pages.
+    const CSSCoord curPos = aAxis.GetRectOffset(visualViewportRect) -
+                            aAxis.GetRectOffset(mMetrics.GetScrollableRect());
+
+    const CSSToOuterCSSScale thumbPosRatio(
+        (maxMinPosDifference != 0)
+            ? float((mScrollbarData.mScrollTrackLength - effectiveThumbLength) /
+                    maxMinPosDifference)
+            : 1.f);
+
+    const OuterCSSCoord desiredThumbPos = curPos * thumbPosRatio;
+
+    translation = desiredThumbPos - mScrollbarData.mThumbStart;
+  } else {
+    // In this branch (taken when apz.scrollthumb.recalc=false), |translation|
+    // and |scale| are computed using the pre-bug1554795 approach of turning
+    // the async scroll and zoom deltas into transforms to apply to the
+    // main-thread thumb position and size.
+
+    // The scroll thumb needs to be scaled in the direction of scrolling by the
+    // inverse of the async zoom. This is because zooming in decreases the
+    // fraction of the whole srollable rect that is in view.
+    scale = 1.f / asyncZoom;
+
+    // Note: |metrics.GetZoom()| doesn't yet include the async zoom.
+    CSSToParentLayerScale effectiveZoom =
+        CSSToParentLayerScale(mMetrics.GetZoom().scale * asyncZoom);
+
+    if (gfxPlatform::UseDesktopZoomingScrollbars()) {
+      // As computed by GetCurrentAsyncTransform, asyncScrollY is
+      //   asyncScrollY = -(GetEffectiveScrollOffset -
+      //   mLastContentPaintMetrics.GetLayoutScrollOffset()) *
+      //   effectiveZoom
+      // where GetEffectiveScrollOffset includes the visual viewport offset that
+      // the main thread knows about plus any async scrolling to the visual
+      // viewport offset that the main thread does not (yet) know about. We want
+      // asyncScrollY to be
+      //   asyncScrollY = -(GetEffectiveScrollOffset -
+      //   mLastContentPaintMetrics.GetVisualScrollOffset()) * effectiveZoom
+      // because the main thread positions the scrollbars at the visual viewport
+      // offset that it knows about. (aMetrics is mLastContentPaintMetrics)
+
+      asyncScroll -= aAxis.GetPointOffset((mMetrics.GetLayoutScrollOffset() -
+                                           mMetrics.GetVisualScrollOffset()) *
+                                          effectiveZoom);
+    }
+
+    // Here we convert the scrollbar thumb ratio into a true unitless ratio by
+    // dividing out the conversion factor from the scrollframe's parent's space
+    // to the scrollframe's space.
+    float unitlessThumbRatio = mScrollbarData.mThumbRatio /
+                               (mMetrics.GetPresShellResolution() * asyncZoom);
+
+    // The scroll thumb needs to be translated in opposite direction of the
+    // async scroll. This is because scrolling down, which translates the layer
+    // content up, should result in moving the scroll thumb down.
+    ParentLayerCoord translationPL = -asyncScroll * unitlessThumbRatio;
+
+    // The translation we computed is in the scroll frame's ParentLayer space.
+    // This includes the full cumulative resolution, even if we are a subframe.
+    // However, the resulting transform is used in a context where the scrollbar
+    // is already subject to the resolutions of enclosing scroll frames. To
+    // avoid double application of these enclosing resolutions, divide them out,
+    // leaving only the local resolution if any.
+    translationPL /= (mMetrics.GetCumulativeResolution().scale /
+                      mMetrics.GetPresShellResolution());
+
+    // Convert translation to CSS pixels as this is what TranslateThumb expects.
+    translation = ViewAs<OuterCSSPixel>(
+        translationPL / (mMetrics.GetDevPixelsPerCSSPixel() *
+                         LayoutDeviceToParentLayerScale(1.0)),
+        PixelCastJustification::CSSPixelsOfSurroundingContent);
+  }
+
+  // When scaling the thumb to account for the async zoom, keep the position
+  // of the start of the thumb (which corresponds to the scroll offset)
+  // constant.
+  if (haveAsyncZoom) {
+    ScaleThumbBy(aAxis, scale, ScrollThumbExtent::Start);
+  }
+
+  // If the page is overscrolled, additionally squish the thumb in accordance
+  // with the overscroll amount.
+  if (overscroll != 0) {
+    float overscrollScale =
+        1.0f - (std::abs(overscroll.value) /
+                aAxis.GetRectLength(mMetrics.GetCompositionBounds()));
+    MOZ_ASSERT(overscrollScale > 0.0f && overscrollScale <= 1.0f);
+    // If we're overscrolled at the top, keep the top of the thumb in place
+    // as we squish it. If we're overscrolled at the bottom, keep the bottom of
+    // the thumb in place.
+    ScaleThumbBy(
+        aAxis, overscrollScale,
+        overscroll < 0 ? ScrollThumbExtent::Start : ScrollThumbExtent::End);
+  }
+
+  TranslateThumb(aAxis, translation);
+}
+
+LayerToParentLayerMatrix4x4 AsyncScrollThumbTransformer::ComputeTransform() {
+  // We only apply the transform if the scroll-target layer has non-container
+  // children (i.e. when it has some possibly-visible content). This is to
+  // avoid moving scroll-bars in the situation that only a scroll information
+  // layer has been built for a scroll frame, as this would result in a
+  // disparity between scrollbars and visible content.
+  if (mMetrics.IsScrollInfoLayer()) {
+    return LayerToParentLayerMatrix4x4{};
+  }
+
+  MOZ_RELEASE_ASSERT(mApzc);
+
+  mAsyncTransform =
+      mApzc->GetCurrentAsyncTransform(AsyncPanZoomController::eForCompositing);
+
+  // |mAsyncTransform| represents the amount by which we have scrolled and
+  // zoomed since the last paint. Because the scrollbar was sized and positioned
+  // based on the painted content, we need to adjust it based on asyncTransform
+  // so that it reflects what the user is actually seeing now.
+  if (*mScrollbarData.mDirection == ScrollDirection::eVertical) {
+    ApplyTransformForAxis(mApzc->mY);
+  }
+  if (*mScrollbarData.mDirection == ScrollDirection::eHorizontal) {
+    ApplyTransformForAxis(mApzc->mX);
+  }
+
+  LayerToParentLayerMatrix4x4 transform =
+      mCurrentTransform * mScrollbarTransform;
+
+  AsyncTransformComponentMatrix compensation;
+  // If the scrollbar layer is a child of the content it is a scrollbar for,
+  // then we need to adjust for any async transform (including an overscroll
+  // transform) on the content. This needs to be cancelled out because layout
+  // positions and sizes the scrollbar on the assumption that there is no async
+  // transform, and without this adjustment the scrollbar will end up in the
+  // wrong place.
+  //
+  // Note that since the async transform is applied on top of the content's
+  // regular transform, we need to make sure to unapply the async transform in
+  // the same coordinate space. This requires applying the content transform
+  // and then unapplying it after unapplying the async transform.
+  if (mScrollbarIsDescendant) {
+    AsyncTransformComponentMatrix overscroll =
+        mApzc->GetOverscrollTransform(AsyncPanZoomController::eForCompositing);
+    gfx::Matrix4x4 asyncUntransform =
+        (mAsyncTransform * overscroll).Inverse().ToUnknownMatrix();
+    const gfx::Matrix4x4& contentTransform = mScrollableContentTransform;
+    gfx::Matrix4x4 contentUntransform = contentTransform.Inverse();
+
+    compensation *= ViewAs<AsyncTransformComponentMatrix>(
+        contentTransform * asyncUntransform * contentUntransform);
+  }
+  transform = transform * compensation;
+
+  return transform;
+}
+
+LayerToParentLayerMatrix4x4 ComputeTransformForScrollThumb(
+    const LayerToParentLayerMatrix4x4& aCurrentTransform,
+    const gfx::Matrix4x4& aScrollableContentTransform,
+    AsyncPanZoomController* aApzc, const FrameMetrics& aMetrics,
+    const ScrollbarData& aScrollbarData, bool aScrollbarIsDescendant) {
+  return AsyncScrollThumbTransformer{
+      aCurrentTransform, aScrollableContentTransform, aApzc, aMetrics,
+      aScrollbarData,    aScrollbarIsDescendant}
+      .ComputeTransform();
+}
+
+}  // namespace apz
+}  // namespace layers
+}  // namespace mozilla