Bug 1753565 - Default disable smooth scrolls if prefers-reduced-motion. r=botond

If the user has the prefers-reduced-motion system setting enabled, do
not enable smooth scrolls by default.

Differential Revision: https://phabricator.services.mozilla.com/D172446

Author: dlrobertson

gfx/layers/apz/test/gtest/TestBasic.cpp

@@ -310,6 +310,7 @@ TEST_F(APZCBasicTester, RelativeScrollOffset) {
 }
 
 TEST_F(APZCBasicTester, MultipleSmoothScrollsSmooth) {
+  SCOPED_GFX_PREF_BOOL("general.smoothScroll", true);
   // We want to test that if we send multiple smooth scroll requests that we
   // still smoothly animate, ie that we get non-zero change every frame while
   // the animation is running.
@@ -450,11 +451,13 @@ class APZCSmoothScrollTester : public APZCBasicTester {
 };
 
 TEST_F(APZCSmoothScrollTester, SmoothScrollDestinationUpdateBezier) {
+  SCOPED_GFX_PREF_BOOL("general.smoothScroll", true);
   SCOPED_GFX_PREF_BOOL("general.smoothScroll.msdPhysics.enabled", false);
   TestSmoothScrollDestinationUpdate();
 }
 
 TEST_F(APZCSmoothScrollTester, SmoothScrollDestinationUpdateMsd) {
+  SCOPED_GFX_PREF_BOOL("general.smoothScroll", true);
   SCOPED_GFX_PREF_BOOL("general.smoothScroll.msdPhysics.enabled", true);
   TestSmoothScrollDestinationUpdate();
 }

layout/base/nsLayoutUtils.cpp

@@ -85,6 +85,7 @@
 #include "mozilla/StaticPrefs_apz.h"
 #include "mozilla/StaticPrefs_dom.h"
 #include "mozilla/StaticPrefs_font.h"
+#include "mozilla/StaticPrefs_general.h"
 #include "mozilla/StaticPrefs_gfx.h"
 #include "mozilla/StaticPrefs_image.h"
 #include "mozilla/StaticPrefs_layers.h"
@@ -7769,6 +7770,22 @@ size_t nsLayoutUtils::SizeOfTextRunsForFrames(nsIFrame* aFrame,
   return total;
 }
 
+/* static */
+void nsLayoutUtils::RecomputeSmoothScrollDefault() {
+  // We want prefers-reduced-motion to determine the default
+  // value of the general.smoothScroll pref. If the user
+  // changed the pref we want to respect the change.
+  Preferences::SetBool(
+      "general.smoothScroll",
+      !LookAndFeel::GetInt(LookAndFeel::IntID::PrefersReducedMotion, 0),
+      PrefValueKind::Default);
+}
+
+/* static */
+bool nsLayoutUtils::IsSmoothScrollingEnabled() {
+  return StaticPrefs::general_smoothScroll();
+}
+
 /* static */
 void nsLayoutUtils::Initialize() {
   nsComputedDOMStyle::RegisterPrefChangeCallbacks();

layout/base/nsLayoutUtils.h

@@ -3039,6 +3039,19 @@ class nsLayoutUtils {
    **/
   static nsIFrame* GetNearestOverflowClipFrame(nsIFrame* aFrame);
 
+  /*
+   * Returns true if the user's preferences allow for smooth scrolling.
+   */
+  static bool IsSmoothScrollingEnabled();
+
+  /*
+   * Recompute the default value of general.smoothScroll based on
+   * the system settings for prefers-reduced-motion.
+   *
+   * Note: Must only be called from the main thread.
+   */
+  static void RecomputeSmoothScrollDefault();
+
  private:
   /**
    * Helper function for LogTestDataForPaint().

layout/generic/nsGfxScrollFrame.cpp

@@ -2234,11 +2234,6 @@ void nsHTMLScrollFrame::AsyncScroll::InitSmoothScroll(
   mAnimationPhysics->Update(aTime, aDestination, aCurrentVelocity);
 }
 
-/* static */
-bool nsHTMLScrollFrame::IsSmoothScrollingEnabled() {
-  return StaticPrefs::general_smoothScroll();
-}
-
 /*
  * Callback function from AsyncSmoothMSDScroll, used in
  * nsHTMLScrollFrame::ScrollTo
@@ -2560,7 +2555,8 @@ void nsHTMLScrollFrame::ScrollToWithOrigin(nsPoint aScrollPosition,
     mAsyncScroll->SetRefreshObserver(this);
   }
 
-  const bool isSmoothScroll = aParams.IsSmooth() && IsSmoothScrollingEnabled();
+  const bool isSmoothScroll =
+      aParams.IsSmooth() && nsLayoutUtils::IsSmoothScrollingEnabled();
   if (isSmoothScroll) {
     mAsyncScroll->InitSmoothScroll(now, GetScrollPosition(), mDestination,
                                    aParams.mOrigin, range, currentVelocity);
@@ -7903,7 +7899,7 @@ bool nsHTMLScrollFrame::IsSmoothScroll(dom::ScrollBehavior aBehavior) const {
   // smooth scrolls. A requested smooth scroll when smooth scrolling is
   // disabled should be equivalent to an instant scroll.
   if (aBehavior == dom::ScrollBehavior::Instant ||
-      !nsHTMLScrollFrame::IsSmoothScrollingEnabled()) {
+      !nsLayoutUtils::IsSmoothScrollingEnabled()) {
     return false;
   }
 

layout/generic/nsGfxScrollFrame.h

@@ -538,8 +538,6 @@ class nsHTMLScrollFrame : public nsContainerFrame,
   void PostScrolledAreaEvent();
   MOZ_CAN_RUN_SCRIPT void FireScrolledAreaEvent();
 
-  static bool IsSmoothScrollingEnabled();
-
   /**
    * @note This method might destroy the frame, pres shell and other objects.
    */

widget/nsXPLookAndFeel.cpp

@@ -17,6 +17,7 @@
 #include "nsFont.h"
 #include "nsIFrame.h"
 #include "nsIXULRuntime.h"
+#include "nsLayoutUtils.h"
 #include "Theme.h"
 #include "SurfaceCacheUtils.h"
 #include "mozilla/dom/ContentParent.h"
@@ -549,6 +550,10 @@ void nsXPLookAndFeel::Init() {
 
   RecomputeColorSchemes();
 
+  if (XRE_IsParentProcess()) {
+    nsLayoutUtils::RecomputeSmoothScrollDefault();
+  }
+
   // XXX If we could reorganize the pref names, we should separate the branch
   //     for each types.  Then, we could reduce the unnecessary loop from
   //     nsXPLookAndFeel::OnPrefChanged().
@@ -1170,8 +1175,9 @@ void nsXPLookAndFeel::RefreshImpl() {
   sIntCache.Clear();
   RecomputeColorSchemes();
 
-  // Clear any cached FullLookAndFeel data, which is now invalid.
   if (XRE_IsParentProcess()) {
+    nsLayoutUtils::RecomputeSmoothScrollDefault();
+    // Clear any cached FullLookAndFeel data, which is now invalid.
     widget::RemoteLookAndFeel::ClearCachedData();
   }
 }