implement MouseEvent.buttons property

Add the `buttons` read-only property to MouseEvent as specified by
the W3C UI Events spec (unsigned short bitmask of currently pressed
buttons). Propagate the field through PointerEvent and WheelEvent
constructors which inherit from MouseEvent.

src/browser/tests/event/mouse.html

@@ -10,11 +10,13 @@
   testing.expectEqual(0, event.clientY);
   testing.expectEqual(0, event.screenX);
   testing.expectEqual(0, event.screenY);
+  testing.expectEqual(0, event.buttons);
 </script>
 
 <script id=parameters>
-  let new_event = new MouseEvent('click', { 'button': 0, 'clientX': 10, 'clientY': 20, screenX: 200, screenY: 500 });
+  let new_event = new MouseEvent('click', { 'button': 0, 'clientX': 10, 'clientY': 20, screenX: 200, screenY: 500, buttons: 5 });
   testing.expectEqual(0, new_event.button);
+  testing.expectEqual(5, new_event.buttons);
   testing.expectEqual(10, new_event.x);
   testing.expectEqual(20, new_event.y);
   testing.expectEqual(10, new_event.pageX);

src/browser/webapi/event/MouseEvent.zig

@@ -48,7 +48,7 @@ _proto: *UIEvent,
 
 _alt_key: bool,
 _button: MouseButton,
-// TODO: _buttons
+_buttons: u16,
 _client_x: f64,
 _client_y: f64,
 _ctrl_key: bool,
@@ -69,7 +69,7 @@ pub const MouseEventOptions = struct {
     altKey: bool = false,
     metaKey: bool = false,
     button: i32 = 0,
-    // TODO: buttons
+    buttons: u16 = 0,
     relatedTarget: ?*EventTarget = null,
 };
 
@@ -100,6 +100,7 @@ pub fn init(typ: []const u8, _opts: ?Options, page: *Page) !*MouseEvent {
             ._alt_key = opts.altKey,
             ._meta_key = opts.metaKey,
             ._button = std.meta.intToEnum(MouseButton, opts.button) catch return error.TypeError,
+            ._buttons = opts.buttons,
             ._related_target = opts.relatedTarget,
         },
     );
@@ -137,6 +138,10 @@ pub fn getButton(self: *const MouseEvent) u8 {
     return @intFromEnum(self._button);
 }
 
+pub fn getButtons(self: *const MouseEvent) u16 {
+    return self._buttons;
+}
+
 pub fn getClientX(self: *const MouseEvent) f64 {
     return self._client_x;
 }
@@ -193,6 +198,7 @@ pub const JsApi = struct {
     pub const constructor = bridge.constructor(MouseEvent.init, .{});
     pub const altKey = bridge.accessor(getAltKey, null, .{});
     pub const button = bridge.accessor(getButton, null, .{});
+    pub const buttons = bridge.accessor(getButtons, null, .{});
     pub const clientX = bridge.accessor(getClientX, null, .{});
     pub const clientY = bridge.accessor(getClientY, null, .{});
     pub const ctrlKey = bridge.accessor(getCtrlKey, null, .{});

src/browser/webapi/event/PointerEvent.zig

@@ -103,6 +103,7 @@ pub fn init(typ: []const u8, _opts: ?Options, page: *Page) !*PointerEvent {
             ._alt_key = opts.altKey,
             ._meta_key = opts.metaKey,
             ._button = std.meta.intToEnum(MouseEvent.MouseButton, opts.button) catch return error.TypeError,
+            ._buttons = opts.buttons,
             ._related_target = opts.relatedTarget,
         },
         PointerEvent{

src/browser/webapi/event/WheelEvent.zig

@@ -70,6 +70,7 @@ pub fn init(typ: []const u8, _opts: ?Options, page: *Page) !*WheelEvent {
             ._alt_key = opts.altKey,
             ._meta_key = opts.metaKey,
             ._button = std.meta.intToEnum(MouseEvent.MouseButton, opts.button) catch return error.TypeError,
+            ._buttons = opts.buttons,
             ._related_target = opts.relatedTarget,
         },
         WheelEvent{