Bug 1675949 - Use a non-fixed value for pixel deltas. r=masayuki

Your call if you think this is worth it. Might need some test
adjustments.

Differential Revision: https://phabricator.services.mozilla.com/D107199

dom/events/EventStateManager.cpp

@@ -2373,18 +2373,9 @@ void EventStateManager::DispatchLegacyMouseScrollEvents(
 
   // Ignore mouse wheel transaction for computing legacy mouse wheel
   // events' delta value.
-  nsIFrame* scrollFrame = ComputeScrollTargetAndMayAdjustWheelEvent(
-      aTargetFrame, aEvent, COMPUTE_LEGACY_MOUSE_SCROLL_EVENT_TARGET);
-
-  nsIScrollableFrame* scrollTarget = do_QueryFrame(scrollFrame);
-  nsPresContext* pc =
-      scrollFrame ? scrollFrame->PresContext() : aTargetFrame->PresContext();
-
   // DOM event's delta vales are computed from CSS pixels.
-  nsSize scrollAmount = GetScrollAmount(pc, aEvent, scrollTarget);
-  nsIntSize scrollAmountInCSSPixels(
-      nsPresContext::AppUnitsToIntCSSPixels(scrollAmount.width),
-      nsPresContext::AppUnitsToIntCSSPixels(scrollAmount.height));
+  auto scrollAmountInCSSPixels =
+      CSSIntSize::FromAppUnits(aEvent->mScrollAmount);
 
   // XXX We don't deal with fractional amount in legacy event, though the
   //     default action handler (DoScrollText()) deals with it.
@@ -5937,6 +5928,15 @@ void EventStateManager::DeltaAccumulator::InitLineOrPageDelta(
   mHandlingDeltaMode = aEvent->mDeltaMode;
   mIsNoLineOrPageDeltaDevice = aEvent->mIsNoLineOrPageDelta;
 
+  {
+    nsIFrame* frame = aESM->ComputeScrollTarget(
+        aTargetFrame, aEvent, COMPUTE_LEGACY_MOUSE_SCROLL_EVENT_TARGET);
+    nsPresContext* pc =
+        frame ? frame->PresContext() : aTargetFrame->PresContext();
+    nsIScrollableFrame* scrollTarget = do_QueryFrame(frame);
+    aEvent->mScrollAmount = aESM->GetScrollAmount(pc, aEvent, scrollTarget);
+  }
+
   // If it's handling neither a device that does not provide line or page deltas
   // nor delta values multiplied by prefs, we must not modify lineOrPageDelta
   // values.
@@ -5969,15 +5969,8 @@ void EventStateManager::DeltaAccumulator::InitLineOrPageDelta(
     // eMouseScrollEventClass (DOMMouseScroll) but not be used for scrolling
     // of default action.  The transaction should be used only for the default
     // action.
-    nsIFrame* frame = aESM->ComputeScrollTarget(
-        aTargetFrame, aEvent, COMPUTE_LEGACY_MOUSE_SCROLL_EVENT_TARGET);
-    nsPresContext* pc =
-        frame ? frame->PresContext() : aTargetFrame->PresContext();
-    nsIScrollableFrame* scrollTarget = do_QueryFrame(frame);
-    nsSize scrollAmount = aESM->GetScrollAmount(pc, aEvent, scrollTarget);
-    nsIntSize scrollAmountInCSSPixels(
-        nsPresContext::AppUnitsToIntCSSPixels(scrollAmount.width),
-        nsPresContext::AppUnitsToIntCSSPixels(scrollAmount.height));
+    auto scrollAmountInCSSPixels =
+        CSSIntSize::FromAppUnits(aEvent->mScrollAmount);
 
     aEvent->mLineOrPageDeltaX = RoundDown(mX) / scrollAmountInCSSPixels.width;
     aEvent->mLineOrPageDeltaY = RoundDown(mY) / scrollAmountInCSSPixels.height;

dom/events/WheelEvent.cpp

@@ -61,7 +61,8 @@ void WheelEvent::InitWheelEvent(
 }
 
 double WheelEvent::ToWebExposedDelta(const WidgetWheelEvent& aWidgetEvent,
-                                     double aDelta, CallerType aCallerType) {
+                                     double aDelta, nscoord aLineOrPageAmount,
+                                     CallerType aCallerType) {
   if (aCallerType != CallerType::System) {
     if (mDeltaModeCheckingState == DeltaModeCheckingState::Unknown) {
       mDeltaModeCheckingState = DeltaModeCheckingState::Unchecked;
@@ -69,9 +70,7 @@ double WheelEvent::ToWebExposedDelta(const WidgetWheelEvent& aWidgetEvent,
     if (mDeltaModeCheckingState == DeltaModeCheckingState::Unchecked &&
         aWidgetEvent.mDeltaMode == WheelEvent_Binding::DOM_DELTA_LINE &&
         StaticPrefs::dom_event_wheel_deltaMode_lines_disabled()) {
-      // TODO(emilio, bug 1675949): Consider not using a fixed multiplier here?
-      return aDelta *
-             StaticPrefs::dom_event_wheel_deltaMode_lines_to_pixel_scale();
+      return aDelta * CSSPixel::FromAppUnits(aLineOrPageAmount);
     }
   }
   if (!mAppUnitsPerDevPixel) {
@@ -82,17 +81,21 @@ double WheelEvent::ToWebExposedDelta(const WidgetWheelEvent& aWidgetEvent,
 
 double WheelEvent::DeltaX(CallerType aCallerType) {
   WidgetWheelEvent* ev = mEvent->AsWheelEvent();
-  return ToWebExposedDelta(*ev, ev->mDeltaX, aCallerType);
+  return ToWebExposedDelta(*ev, ev->mDeltaX, ev->mScrollAmount.width,
+                           aCallerType);
 }
 
 double WheelEvent::DeltaY(CallerType aCallerType) {
   WidgetWheelEvent* ev = mEvent->AsWheelEvent();
-  return ToWebExposedDelta(*ev, ev->mDeltaY, aCallerType);
+  return ToWebExposedDelta(*ev, ev->mDeltaY, ev->mScrollAmount.height,
+                           aCallerType);
 }
 
 double WheelEvent::DeltaZ(CallerType aCallerType) {
   WidgetWheelEvent* ev = mEvent->AsWheelEvent();
-  return ToWebExposedDelta(*ev, ev->mDeltaZ, aCallerType);
+  // XXX Unclear what scroll amount we should use for deltaZ...
+  auto amount = std::max(ev->mScrollAmount.width, ev->mScrollAmount.height);
+  return ToWebExposedDelta(*ev, ev->mDeltaZ, amount, aCallerType);
 }
 
 uint32_t WheelEvent::DeltaMode(CallerType aCallerType) {

dom/events/WheelEvent.h

@@ -49,7 +49,8 @@ class WheelEvent : public MouseEvent {
  protected:
   ~WheelEvent() = default;
 
-  double ToWebExposedDelta(const WidgetWheelEvent&, double aDelta, CallerType);
+  double ToWebExposedDelta(const WidgetWheelEvent&, double aDelta,
+                           nscoord aLineOrPageAmount, CallerType);
 
  private:
   int32_t mAppUnitsPerDevPixel;

dom/events/test/test_continuous_wheel_events.html

@@ -2503,9 +2503,9 @@ function* testContinuousTrustedEvents()
       wheel: {
         expected: true, preventDefault: false,
         skipDeltaModeCheck: true,
-        deltaX: SpecialPowers.getIntPref("dom.event.wheel-deltaMode-lines-to-pixel-scale"),
-        deltaY: SpecialPowers.getIntPref("dom.event.wheel-deltaMode-lines-to-pixel-scale"),
-        deltaZ: SpecialPowers.getIntPref("dom.event.wheel-deltaMode-lines-to-pixel-scale"),
+        deltaX: gHorizontalLine,
+        deltaY: gLineHeight,
+        deltaZ: Math.max(gHorizontalLine, gLineHeight),
       },
       DOMMouseScroll: {
         horizontal: { expected: true,  preventDefault: false, detail: 1 },

modules/libpref/init/StaticPrefList.yaml

@@ -1780,15 +1780,6 @@
   value: false
   mirror: always
 
-# When dom.event.wheel-deltaMode-lines.disabled is true, this is the lines to
-# pixels multiplier that gets used.
-#
-# The value here is taken from PIXELS_PER_LINE_SCALE from pdf.js.
-- name: dom.event.wheel-deltaMode-lines-to-pixel-scale
-  type: uint32_t
-  value: 30
-  mirror: always
-
 #if defined(XP_MACOSX)
 # Whether to disable treating ctrl click as right click
 - name: dom.event.treat_ctrl_click_as_right_click.disabled

widget/MouseEvents.h

@@ -513,6 +513,12 @@ class WidgetWheelEvent : public WidgetMouseEventBase {
   double mDeltaY;
   double mDeltaZ;
 
+  // The amount of scrolling per line or page, without accounting for mouse
+  // wheel transactions etc.
+  //
+  // Computed by EventStateManager::DeltaAccumulator::InitLineOrPageDelta.
+  nsSize mScrollAmount;
+
   // overflowed delta values for scroll, these values are set by
   // EventStateManger.  If the default action of the wheel event isn't scroll,
   // these values are always zero.  Otherwise, remaining delta values which are
@@ -609,6 +615,7 @@ class WidgetWheelEvent : public WidgetMouseEventBase {
     mDeltaY = aEvent.mDeltaY;
     mDeltaZ = aEvent.mDeltaZ;
     mDeltaMode = aEvent.mDeltaMode;
+    mScrollAmount = aEvent.mScrollAmount;
     mCustomizedByUserPrefs = aEvent.mCustomizedByUserPrefs;
     mMayHaveMomentum = aEvent.mMayHaveMomentum;
     mIsMomentum = aEvent.mIsMomentum;