Integer coordinates for click/contextmenu as pointerevents

There is a potential compat issue with making
click/contextmenu pointer events. The coordinates
(clientX/clientY, screenX/screenY, pageX/pageY,
offsetX/offsetY) will be returned as fractional numbers.

This is a mitigation plan for the unlikely case of breakage from
fractional coordinates. We believe this is unlikely because we
have been running a Finch experiment with ClickPointerEvent flag
enabled in Canary+Dev for 2 months and there were no related bug
reports.

Our mitigation plan would allow us to keep click/contextmenu
as pointerevents but make sure to return integer
instead of fractional coordinates.

Bug: 989958
Change-Id: I190010ab65f1e01b58480973660b4823f51a24fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2510931
Commit-Queue: Liviu Tinta <liviutinta@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828207}

third_party/blink/renderer/core/events/pointer_event.cc

@@ -100,11 +100,22 @@ bool PointerEvent::IsMouseEvent() const {
   return false;
 }
 
+bool PointerEvent::ShouldHaveIntegerCoordinates() const {
+  if (RuntimeEnabledFeatures::ClickPointerEventIntegerCoordinatesEnabled() &&
+      (type() == event_type_names::kClick ||
+       type() == event_type_names::kContextmenu)) {
+    return true;
+  }
+  return false;
+}
+
 bool PointerEvent::IsPointerEvent() const {
   return true;
 }
 
 double PointerEvent::offsetX() const {
+  if (ShouldHaveIntegerCoordinates())
+    return MouseEvent::offsetX();
   if (!HasPosition())
     return 0;
   if (!has_cached_relative_position_)
@@ -113,6 +124,8 @@ double PointerEvent::offsetX() const {
 }
 
 double PointerEvent::offsetY() const {
+  if (ShouldHaveIntegerCoordinates())
+    return MouseEvent::offsetY();
   if (!HasPosition())
     return 0;
   if (!has_cached_relative_position_)

third_party/blink/renderer/core/events/pointer_event.h

@@ -7,6 +7,7 @@
 
 #include "third_party/blink/public/common/input/pointer_id.h"
 #include "third_party/blink/renderer/core/events/mouse_event.h"
+#include "third_party/blink/renderer/platform/runtime_enabled_features.h"
 #include "third_party/blink/renderer/platform/wtf/casting.h"
 
 namespace blink {
@@ -56,12 +57,36 @@ class CORE_EXPORT PointerEvent final : public MouseEvent {
   bool IsMouseEvent() const override;
   bool IsPointerEvent() const override;
 
-  double screenX() const override { return screen_location_.X(); }
-  double screenY() const override { return screen_location_.Y(); }
-  double clientX() const override { return client_location_.X(); }
-  double clientY() const override { return client_location_.Y(); }
-  double pageX() const override { return page_location_.X(); }
-  double pageY() const override { return page_location_.Y(); }
+  double screenX() const override {
+    if (ShouldHaveIntegerCoordinates())
+      return MouseEvent::screenX();
+    return screen_location_.X();
+  }
+  double screenY() const override {
+    if (ShouldHaveIntegerCoordinates())
+      return MouseEvent::screenY();
+    return screen_location_.Y();
+  }
+  double clientX() const override {
+    if (ShouldHaveIntegerCoordinates())
+      return MouseEvent::clientX();
+    return client_location_.X();
+  }
+  double clientY() const override {
+    if (ShouldHaveIntegerCoordinates())
+      return MouseEvent::clientY();
+    return client_location_.Y();
+  }
+  double pageX() const override {
+    if (ShouldHaveIntegerCoordinates())
+      return MouseEvent::pageX();
+    return page_location_.X();
+  }
+  double pageY() const override {
+    if (ShouldHaveIntegerCoordinates())
+      return MouseEvent::pageY();
+    return page_location_.Y();
+  }
 
   double offsetX() const override;
   double offsetY() const override;
@@ -82,6 +107,8 @@ class CORE_EXPORT PointerEvent final : public MouseEvent {
   void Trace(Visitor*) const override;
 
  private:
+  bool ShouldHaveIntegerCoordinates() const;
+
   PointerId pointer_id_;
   double width_;
   double height_;

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -337,6 +337,9 @@
       name: "ClickPointerEvent",
       status: "experimental",
     },
+    {
+      name: "ClickPointerEventIntegerCoordinates",
+    },
     {
       name: "ClickRetargetting",
       status: "experimental",