Bug 1584859 - Make smooth scrolling conditional on reduced motion settings. r=dbaron

We could put this change itself behind a pref too, if we considered that worth
it. But probably not so.

Differential Revision: https://phabricator.services.mozilla.com/D48010

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/src/APZInputBridge.cpp

@@ -19,6 +19,7 @@
 #include "mozilla/TouchEvents.h"          // for WidgetTouchEvent
 #include "mozilla/WheelHandlingHelper.h"  // for WheelDeltaHorizontalizer,
                                           //     WheelDeltaAdjustmentStrategy
+#include "nsLayoutUtils.h"                // for IsSmoothScrollingEnabled
 
 namespace mozilla {
 namespace layers {
@@ -116,7 +117,7 @@ APZEventResult APZInputBridge::ReceiveInputEvent(WidgetInputEvent& aEvent) {
       if (Maybe<APZWheelAction> action = ActionForWheelEvent(&wheelEvent)) {
         ScrollWheelInput::ScrollMode scrollMode =
             ScrollWheelInput::SCROLLMODE_INSTANT;
-        if (StaticPrefs::general_smoothScroll() &&
+        if (nsLayoutUtils::IsSmoothScrollingEnabled() &&
             ((wheelEvent.mDeltaMode ==
                   dom::WheelEvent_Binding::DOM_DELTA_LINE &&
               StaticPrefs::general_smoothScroll_mouseWheel()) ||

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2006,7 +2006,7 @@ nsEventStatus AsyncPanZoomController::OnKeyboard(const KeyboardInput& aEvent) {
       MaybeAdjustDestinationForScrollSnapping(aEvent, destination);
 
   // If smooth scrolling is disabled, then scroll immediately to the destination
-  if (!StaticPrefs::general_smoothScroll()) {
+  if (!nsLayoutUtils::IsSmoothScrollingEnabled()) {
     CancelAnimation();
 
     ParentLayerPoint startPoint, endPoint;

layout/base/nsLayoutUtils.cpp

@@ -27,6 +27,7 @@
 #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_layers.h"
 #include "mozilla/StaticPrefs_layout.h"
@@ -7976,9 +7977,27 @@ size_t nsLayoutUtils::SizeOfTextRunsForFrames(nsIFrame* aFrame,
   return total;
 }
 
+RelaxedAtomicBool nsLayoutUtils::gSmoothScrollingEnabled;
+
+void nsLayoutUtils::RecomputeSmoothScrollingEnabled() {
+  MOZ_RELEASE_ASSERT(NS_IsMainThread(),
+                     "You don't really want to call into widget "
+                     "off the main thread, do you?");
+  gSmoothScrollingEnabled =
+      StaticPrefs::general_smoothScroll_DoNotUseDirectly() &&
+      !LookAndFeel::GetInt(LookAndFeel::eIntID_PrefersReducedMotion, 0);
+}
+
+static void SmoothScrollPrefChanged(const char*, void*) {
+  nsLayoutUtils::RecomputeSmoothScrollingEnabled();
+}
+
 /* static */
 void nsLayoutUtils::Initialize() {
   nsComputedDOMStyle::RegisterPrefChangeCallbacks();
+  Preferences::RegisterCallbackAndCall(
+      SmoothScrollPrefChanged,
+      nsDependentCString(StaticPrefs::GetPrefName_general_smoothScroll()));
 }
 
 /* static */

layout/base/nsLayoutUtils.h

@@ -2998,7 +2998,16 @@ class nsLayoutUtils {
   static bool FrameIsMostlyScrolledOutOfViewInCrossProcess(
       const nsIFrame* aFrame, nscoord aMargin);
 
+  static bool IsSmoothScrollingEnabled() {
+    return gSmoothScrollingEnabled;
+  }
+
+  static void RecomputeSmoothScrollingEnabled();
+
  private:
+
+  static mozilla::RelaxedAtomicBool gSmoothScrollingEnabled;
+
   /**
    * Helper function for LogTestDataForPaint().
    */

layout/generic/nsGfxScrollFrame.cpp

@@ -2026,10 +2026,6 @@ void ScrollFrameHelper::AsyncScroll::InitSmoothScroll(
   mAnimationPhysics->Update(aTime, aDestination, aCurrentVelocity);
 }
 
-bool ScrollFrameHelper::IsSmoothScrollingEnabled() {
-  return StaticPrefs::general_smoothScroll();
-}
-
 class ScrollFrameActivityTracker final
     : public nsExpirationTracker<ScrollFrameHelper, 4> {
  public:
@@ -2347,7 +2343,7 @@ void ScrollFrameHelper::ScrollToWithOrigin(
           ? presContext->RefreshDriver()->MostRecentRefresh()
           : TimeStamp::Now();
   bool isSmoothScroll =
-      aMode == ScrollMode::Smooth && IsSmoothScrollingEnabled();
+      aMode == ScrollMode::Smooth && nsLayoutUtils::IsSmoothScrollingEnabled();
 
   nsSize currentVelocity(0, 0);
 
@@ -7188,6 +7184,10 @@ bool ScrollFrameHelper::SmoothScrollVisual(
 }
 
 bool ScrollFrameHelper::IsSmoothScroll(dom::ScrollBehavior aBehavior) const {
+  if (!nsLayoutUtils::IsSmoothScrollingEnabled()) {
+    return false;
+  }
+
   if (aBehavior == dom::ScrollBehavior::Smooth) {
     return true;
   }

layout/generic/nsGfxScrollFrame.h

@@ -116,8 +116,6 @@ class ScrollFrameHelper : public nsIReflowCallback {
   void PostScrolledAreaEvent();
   void FireScrolledAreaEvent();
 
-  bool IsSmoothScrollingEnabled();
-
   /**
    * @note This method might destroy the frame, pres shell and other objects.
    */

modules/libpref/init/StaticPrefList.yaml

@@ -2959,10 +2959,12 @@
   value: true
   mirror: always
 
+# Use nsLayoutUtils::SmoothScrollingEnabled() instead.
 - name: general.smoothScroll
   type: RelaxedAtomicBool
   value: true
   mirror: always
+  do_not_use_directly: true
 
 # This pref and general.smoothScroll.stopDecelerationWeighting determine
 # the timing function.

widget/cocoa/nsChildView.mm

@@ -4705,7 +4705,8 @@ static gfx::IntPoint GetIntegerDeltaForEvent(NSEvent* aEvent) {
     geckoChildDeathGrip->DispatchAPZWheelInputEvent(wheelEvent, false);
   } else {
     ScrollWheelInput::ScrollMode scrollMode = ScrollWheelInput::SCROLLMODE_INSTANT;
-    if (StaticPrefs::general_smoothScroll() && StaticPrefs::general_smoothScroll_mouseWheel()) {
+    if (nsLayoutUtils::IsSmoothScrollingEnabled() &&
+        StaticPrefs::general_smoothScroll_mouseWheel()) {
       scrollMode = ScrollWheelInput::SCROLLMODE_SMOOTH;
     }
     ScrollWheelInput wheelEvent(eventIntervalTime, eventTimeStamp, modifiers, scrollMode,

widget/nsXPLookAndFeel.cpp

@@ -11,6 +11,7 @@
 #include "nsLookAndFeel.h"
 #include "HeadlessLookAndFeel.h"
 #include "nsCRT.h"
+#include "nsLayoutUtils.h"
 #include "nsFont.h"
 #include "mozilla/dom/ContentChild.h"
 #include "mozilla/Preferences.h"
@@ -993,6 +994,8 @@ void nsXPLookAndFeel::RefreshImpl() {
   for (i = 0; i < uint32_t(ColorID::End); ++i) {
     InitColorFromPref(i);
   }
+
+  nsLayoutUtils::RecomputeSmoothScrollingEnabled();
 }
 
 nsTArray<LookAndFeelInt> nsXPLookAndFeel::GetIntCacheImpl() {