MacViews: Support continuous scrolling and horizontal scrolling

Add support for continuous devices (Magic Mouse and trackpad) using NSEvent
APIs introduced in 10.7.
Tweak ScrollView/ScrollBar to allow horizontal scrolling from MouseWheel
events.

BUG=363529

Review URL: https://codereview.chromium.org/456913002

Cr-Commit-Position: refs/heads/master@{#293038}

base/mac/sdk_forward_declarations.h

@@ -70,6 +70,7 @@ typedef NSUInteger NSWindowButton;
 
 - (NSEventPhase)momentumPhase;
 - (NSEventPhase)phase;
+- (BOOL)hasPreciseScrollingDeltas;
 - (CGFloat)scrollingDeltaX;
 - (CGFloat)scrollingDeltaY;
 - (void)trackSwipeEventWithOptions:(NSEventSwipeTrackingOptions)options

ui/events/cocoa/events_mac.mm

@@ -6,6 +6,8 @@
 
 #include <Cocoa/Cocoa.h>
 
+#import "base/mac/mac_util.h"
+#import "base/mac/sdk_forward_declarations.h"
 #include "base/logging.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
@@ -130,14 +132,24 @@ int GetChangedMouseButtonFlagsFromNative(
 }
 
 gfx::Vector2d GetMouseWheelOffset(const base::NativeEvent& event) {
-  // Empirically, a value of 0.1 is typical for one mousewheel click. Positive
-  // values when scrolling up or to the left. Scrolling quickly results in a
-  // higher delta per click, up to about 15.0. (Quartz documentation suggests
-  // +/-10).
-  // Multiply by 1000 to vaguely approximate WHEEL_DELTA on Windows (120).
-  const CGFloat kWheelDeltaMultiplier = 1000;
-  return gfx::Vector2d(kWheelDeltaMultiplier * [event deltaX],
-                       kWheelDeltaMultiplier * [event deltaY]);
+  if ([event respondsToSelector:@selector(hasPreciseScrollingDeltas)] &&
+      [event hasPreciseScrollingDeltas]) {
+    // Handle continuous scrolling devices such as a Magic Mouse or a trackpad.
+    // -scrollingDelta{X|Y} have float return types but they return values that
+    // are already rounded to integers.
+    // The values are the same as the values returned from calling
+    // CGEventGetIntegerValueField(kCGScrollWheelEventPointDeltaAxis{1|2}).
+    return gfx::Vector2d([event scrollingDeltaX], [event scrollingDeltaY]);
+  } else {
+    // Empirically, a value of 0.1 is typical for one mousewheel click. Positive
+    // values when scrolling up or to the left. Scrolling quickly results in a
+    // higher delta per click, up to about 15.0. (Quartz documentation suggests
+    // +/-10).
+    // Multiply by 1000 to vaguely approximate WHEEL_DELTA on Windows (120).
+    const CGFloat kWheelDeltaMultiplier = 1000;
+    return gfx::Vector2d(kWheelDeltaMultiplier * [event deltaX],
+                         kWheelDeltaMultiplier * [event deltaY]);
+  }
 }
 
 base::NativeEvent CopyNativeEvent(const base::NativeEvent& event) {

ui/views/cocoa/bridged_content_view.mm

@@ -149,7 +149,11 @@
 }
 
 - (void)scrollWheel:(NSEvent*)theEvent {
-  [self handleMouseEvent:theEvent];
+  if (!hostedView_)
+    return;
+
+  ui::MouseWheelEvent event(theEvent);
+  hostedView_->GetWidget()->OnMouseEvent(&event);
 }
 
 - (void)deleteBackward:(id)sender {

ui/views/controls/scroll_view.cc

@@ -354,12 +354,12 @@ bool ScrollView::OnKeyPressed(const ui::KeyEvent& event) {
 
 bool ScrollView::OnMouseWheel(const ui::MouseWheelEvent& e) {
   bool processed = false;
-  // Give vertical scrollbar priority
+
   if (vert_sb_->visible())
     processed = vert_sb_->OnMouseWheel(e);
 
-  if (!processed && horiz_sb_->visible())
-    processed = horiz_sb_->OnMouseWheel(e);
+  if (horiz_sb_->visible())
+    processed = horiz_sb_->OnMouseWheel(e) || processed;
 
   return processed;
 }

ui/views/controls/scrollbar/base_scroll_bar.cc

@@ -194,7 +194,7 @@ bool BaseScrollBar::OnKeyPressed(const ui::KeyEvent& event) {
 }
 
 bool BaseScrollBar::OnMouseWheel(const ui::MouseWheelEvent& event) {
-  ScrollByContentsOffset(event.y_offset());
+  OnScroll(event.x_offset(), event.y_offset());
   return true;
 }