Bug 1392460 - Add a couple prefs to expose DOM_DELTA_PIXEL rather than DOM_DELTA_LINES to the web. r=masayuki,smaug

And use them to disable DOM_DELTA_LINES on Nightly builds, to see the
impact, since this is pretty hard to measure just with telemetry.

Differential Revision: https://phabricator.services.mozilla.com/D95388

dom/events/WheelEvent.cpp

@@ -18,6 +18,11 @@ WheelEvent::WheelEvent(EventTarget* aOwner, nsPresContext* aPresContext,
                      ? aWheelEvent
                      : new WidgetWheelEvent(false, eVoidEvent, nullptr)),
       mAppUnitsPerDevPixel(0) {
+
+  if (StaticPrefs::dom_event_wheel_deltaMode_lines_always_disabled()) {
+    mDeltaModeCheckingState = DeltaModeCheckingState::Unchecked;
+  }
+
   if (aWheelEvent) {
     mEventIsInternal = false;
     // If the delta mode is pixel, the WidgetWheelEvent's delta values are in
@@ -55,31 +60,55 @@ void WheelEvent::InitWheelEvent(
   wheelEvent->mDeltaMode = aDeltaMode;
 }
 
-double WheelEvent::DeltaX() {
-  if (!mAppUnitsPerDevPixel) {
-    return mEvent->AsWheelEvent()->mDeltaX;
+double WheelEvent::ToWebExposedDelta(const WidgetWheelEvent& aWidgetEvent,
+                                     double aDelta, CallerType aCallerType) {
+  if (aCallerType != CallerType::System) {
+    if (mDeltaModeCheckingState == DeltaModeCheckingState::Unknown) {
+      mDeltaModeCheckingState = DeltaModeCheckingState::Unchecked;
+    }
+    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 mEvent->AsWheelEvent()->mDeltaX * mAppUnitsPerDevPixel /
-         AppUnitsPerCSSPixel();
+  if (!mAppUnitsPerDevPixel) {
+    return aDelta;
+  }
+  return aDelta * mAppUnitsPerDevPixel / AppUnitsPerCSSPixel();
 }
 
-double WheelEvent::DeltaY() {
-  if (!mAppUnitsPerDevPixel) {
-    return mEvent->AsWheelEvent()->mDeltaY;
-  }
-  return mEvent->AsWheelEvent()->mDeltaY * mAppUnitsPerDevPixel /
-         AppUnitsPerCSSPixel();
+double WheelEvent::DeltaX(CallerType aCallerType) {
+  WidgetWheelEvent* ev = mEvent->AsWheelEvent();
+  return ToWebExposedDelta(*ev, ev->mDeltaX, aCallerType);
 }
 
-double WheelEvent::DeltaZ() {
-  if (!mAppUnitsPerDevPixel) {
-    return mEvent->AsWheelEvent()->mDeltaZ;
-  }
-  return mEvent->AsWheelEvent()->mDeltaZ * mAppUnitsPerDevPixel /
-         AppUnitsPerCSSPixel();
+double WheelEvent::DeltaY(CallerType aCallerType) {
+  WidgetWheelEvent* ev = mEvent->AsWheelEvent();
+  return ToWebExposedDelta(*ev, ev->mDeltaY, aCallerType);
 }
 
-uint32_t WheelEvent::DeltaMode() { return mEvent->AsWheelEvent()->mDeltaMode; }
+double WheelEvent::DeltaZ(CallerType aCallerType) {
+  WidgetWheelEvent* ev = mEvent->AsWheelEvent();
+  return ToWebExposedDelta(*ev, ev->mDeltaZ, aCallerType);
+}
+
+uint32_t WheelEvent::DeltaMode(CallerType aCallerType) {
+  uint32_t mode = mEvent->AsWheelEvent()->mDeltaMode;
+  if (aCallerType != CallerType::System) {
+    if (mDeltaModeCheckingState == DeltaModeCheckingState::Unknown) {
+      mDeltaModeCheckingState = DeltaModeCheckingState::Checked;
+    } else if (mDeltaModeCheckingState == DeltaModeCheckingState::Unchecked &&
+               mode == WheelEvent_Binding::DOM_DELTA_LINE &&
+               StaticPrefs::dom_event_wheel_deltaMode_lines_disabled()) {
+      return WheelEvent_Binding::DOM_DELTA_PIXEL;
+    }
+  }
+
+  return mode;
+}
 
 already_AddRefed<WheelEvent> WheelEvent::Constructor(
     const GlobalObject& aGlobal, const nsAString& aType,

dom/events/WheelEvent.h

@@ -33,10 +33,10 @@ class WheelEvent : public MouseEvent {
   // NOTE: DeltaX(), DeltaY() and DeltaZ() return CSS pixels when deltaMode is
   //       DOM_DELTA_PIXEL. (The internal event's delta values are device pixels
   //       if it's dispatched by widget)
-  double DeltaX();
-  double DeltaY();
-  double DeltaZ();
-  uint32_t DeltaMode();
+  double DeltaX(CallerType);
+  double DeltaY(CallerType);
+  double DeltaZ(CallerType);
+  uint32_t DeltaMode(CallerType);
 
   void InitWheelEvent(const nsAString& aType, bool aCanBubble, bool aCancelable,
                       nsGlobalWindowInner* aView, int32_t aDetail,
@@ -49,8 +49,26 @@ class WheelEvent : public MouseEvent {
  protected:
   ~WheelEvent() = default;
 
+  double ToWebExposedDelta(const WidgetWheelEvent&, double aDelta, CallerType);
+
  private:
   int32_t mAppUnitsPerDevPixel;
+  enum class DeltaModeCheckingState : uint8_t {
+    // Neither deltaMode nor the delta values have been accessed.
+    Unknown,
+    // The delta values have been accessed, without checking deltaMode first.
+    Unchecked,
+    // The deltaMode has been checked.
+    Checked,
+  };
+
+  // For compat reasons, we might expose a DOM_DELTA_LINE event as
+  // DOM_DELTA_PIXEL instead. Whether we do that depends on whether the event
+  // has been asked for the deltaMode before the deltas. If it has, we assume
+  // that the page will correctly handle DOM_DELTA_LINE. This variable tracks
+  // that state. See bug 1392460.
+  DeltaModeCheckingState mDeltaModeCheckingState =
+      DeltaModeCheckingState::Unknown;
 };
 
 }  // namespace dom

dom/events/test/test_continuous_wheel_events.html

@@ -2494,6 +2494,26 @@ function* testContinuousTrustedEvents()
         horizontal: { expected: true,  preventDefault: true,  detail: gHorizontalLine },
         vertical:   { expected: true,  preventDefault: true,  detail: gLineHeight } },
     },
+    { description: "modifier key tests without content checking mode (alt, line)",
+      event: { deltaMode: WheelEvent.DOM_DELTA_LINE,
+               deltaX: 1.0, deltaY: 1.0, deltaZ: 1.0, isMomentum: false,
+               lineOrPageDeltaX: 1, lineOrPageDeltaY: 1, isNoLineOrPageDelta: false,
+               isCustomizedByPrefs: false,
+               shiftKey: false, ctrlKey: false, altKey: true, metaKey: false },
+      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"),
+      },
+      DOMMouseScroll: {
+        horizontal: { expected: true,  preventDefault: false, detail: 1 },
+        vertical:   { expected: true,  preventDefault: false, detail: 1 } },
+      MozMousePixelScroll: {
+        horizontal: { expected: true,  preventDefault: true,  detail: gHorizontalLine },
+        vertical:   { expected: true,  preventDefault: true,  detail: gLineHeight } },
+    },
     { description: "modifier key tests (alt, page)",
       event: { deltaMode: WheelEvent.DOM_DELTA_PAGE,
                deltaX: 1.0, deltaY: 1.0, deltaZ: 1.0, isMomentum: false,
@@ -3058,14 +3078,24 @@ function* testContinuousTrustedEvents()
 
     is(aEvent.target, gScrolledElement,
        description + "target was invalid");
-    is(aEvent.deltaMode, currentWheelEventTest.event.deltaMode,
-       description + "deltaMode was invalid");
-    is(aEvent.deltaX, currentWheelEventTest.wheel.deltaX,
-       description + "deltaX was invalid");
-    is(aEvent.deltaY, currentWheelEventTest.wheel.deltaY,
-       description + "deltaY was invalid");
-    is(aEvent.deltaZ, currentWheelEventTest.wheel.deltaZ,
-       description + "deltaZ was invalid");
+    if (!currentWheelEventTest.wheel.skipDeltaModeCheck) {
+      is(aEvent.deltaMode, currentWheelEventTest.event.deltaMode,
+         description + "deltaMode was invalid");
+    }
+    is(SpecialPowers.wrap(aEvent).deltaMode, currentWheelEventTest.event.deltaMode,
+       description + "deltaMode is raw value from privileged script");
+    for (let prop of ["deltaX", "deltaY", "deltaZ"]) {
+      is(aEvent[prop], currentWheelEventTest.wheel[prop],
+         description + prop + " was invalid");
+      if (currentWheelEventTest.wheel.skipDeltaModeCheck) {
+        is(aEvent.deltaMode, WheelEvent.DOM_DELTA_PIXEL,
+           description + "deltaMode should become pixels for line scrolling if unchecked by content")
+        if (aEvent[prop] != 0) {
+          isnot(aEvent[prop], SpecialPowers.wrap(aEvent)[prop],
+                description + "should keep returning raw value for privileged script");
+        }
+      }
+    }
     is(aEvent.shiftKey, currentWheelEventTest.event.shiftKey,
        description + "shiftKey was invalid");
     is(aEvent.ctrlKey, currentWheelEventTest.event.ctrlKey,
@@ -3225,6 +3255,12 @@ function* testBody()
 function runTests()
 {
   SpecialPowers.pushPrefEnv({"set": [
+    // FIXME(emilio): This test is broken in HiDPI, unclear if
+    // MozMousePixelScroll is not properly converting to CSS pixels, or
+    // whether sendWheelAndWait expectes device rather than CSS pixels, or
+    // something else.
+    ["layout.css.devPixelsPerPx", 1.0],
+
     ["mousewheel.transaction.timeout", 100000],
     ["mousewheel.default.delta_multiplier_x", 100],
     ["mousewheel.default.delta_multiplier_y", 100],

dom/events/test/test_dom_wheel_event.html

@@ -183,6 +183,8 @@ function* testDeltaMultiplierPrefs()
       deltaX: -gHorizontalLine, deltaY: -gLineHeight, deltaZ: -gLineHeight, lineOrPageDeltaX: -1, lineOrPageDeltaY: -1 },
     { deltaMode: WheelEvent.DOM_DELTA_LINE,
       deltaX: -1.0, deltaY: -1.0, deltaZ: -1.0, lineOrPageDeltaX: -1, lineOrPageDeltaY: -1 },
+    { deltaMode: WheelEvent.DOM_DELTA_LINE, skipDeltaModeCheck: true,
+      deltaX: -1.0, deltaY: -1.0, deltaZ: -1.0, lineOrPageDeltaX: -1, lineOrPageDeltaY: -1 },
     { deltaMode: WheelEvent.DOM_DELTA_PAGE,
       deltaX: -1.0, deltaY: -1.0, deltaZ: -1.0, lineOrPageDeltaX: -1, lineOrPageDeltaY: -1 },
   ];
@@ -222,9 +224,20 @@ function* testDeltaMultiplierPrefs()
           break;
       }
     }
+    if (currentEvent.skipDeltaModeCheck) {
+      let linesToPixel = SpecialPowers.getIntPref("dom.event.wheel-deltaMode-lines-to-pixel-scale");
+      expectedDeltaX *= linesToPixel;
+      expectedDeltaY *= linesToPixel;
+      expectedDeltaZ *= linesToPixel;
+    } else {
+      is(aEvent.deltaMode, currentEvent.deltaMode, description + "deltaMode (" + currentEvent.deltaMode + ") was invalid");
+    }
     is(aEvent.deltaX, expectedDeltaX, description + "deltaX (" + currentEvent.deltaX + ") was invalid");
     is(aEvent.deltaY, expectedDeltaY, description + "deltaY (" + currentEvent.deltaY + ") was invalid");
     is(aEvent.deltaZ, expectedDeltaZ, description + "deltaZ (" + currentEvent.deltaZ + ") was invalid");
+    if (currentEvent.skipDeltaModeCheck) {
+      isnot(SpecialPowers.wrap(aEvent).deltaMode, aEvent.deltaMode, description + "deltaMode should be changed for content if unchecked");
+    }
 
     if (expectedAsyncHandlerCalls > 0 && --expectedAsyncHandlerCalls == 0) {
       setTimeout(continueTest, 0);
@@ -785,6 +798,12 @@ function continueTest()
 function runTest()
 {
   SpecialPowers.pushPrefEnv({"set": [
+    // FIXME(emilio): This test is broken in HiDPI, unclear if
+    // MozMousePixelScroll is not properly converting to CSS pixels, or
+    // whether sendWheelAndWait expectes device rather than CSS pixels, or
+    // something else.
+    ["layout.css.devPixelsPerPx", 1.0],
+
     ["mousewheel.default.delta_multiplier_x", 100],
     ["mousewheel.default.delta_multiplier_y", 100],
     ["mousewheel.default.delta_multiplier_z", 100],

dom/webidl/WheelEvent.webidl

@@ -19,11 +19,10 @@ interface WheelEvent : MouseEvent
   const unsigned long DOM_DELTA_LINE  = 0x01;
   const unsigned long DOM_DELTA_PAGE  = 0x02;
 
-  readonly attribute double        deltaX;
-  readonly attribute double        deltaY;
-  readonly attribute double        deltaZ;
-  readonly attribute unsigned long deltaMode;
-
+  [NeedsCallerType] readonly attribute double        deltaX;
+  [NeedsCallerType] readonly attribute double        deltaY;
+  [NeedsCallerType] readonly attribute double        deltaZ;
+  [NeedsCallerType] readonly attribute unsigned long deltaMode;
 };
 
 dictionary WheelEventInit : MouseEventInit

modules/libpref/init/StaticPrefList.yaml

@@ -1670,6 +1670,33 @@
   value: true
   mirror: always
 
+# Whether WheelEvent should return pixels instead of lines for
+# WheelEvent.deltaX/Y/Z, when deltaMode hasn't been checked.
+#
+# Other browsers don't use line deltas and websites forget to check for it, see
+# bug 1392460.
+- name: dom.event.wheel-deltaMode-lines.disabled
+  type: bool
+  value: @IS_NIGHTLY_BUILD@
+  mirror: always
+
+# Mostly for debugging. Whether we should do the same as
+# dom.event.wheel-deltaMode-lines.disabled, but unconditionally rather than
+# only when deltaMode hasn't been checked.
+- name: dom.event.wheel-deltaMode-lines.always-disabled
+  type: bool
+  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