Handle horizontal wheel input (Windows) (#792)

* add handler for horizontal wheel input

* add changlelog message re now handling horiz scroll on windows

CHANGELOG.md

@@ -45,6 +45,7 @@
 - Refactored Windows state/flag-setting code.
 - On Windows, hiding the cursor no longer hides the cursor for all Winit windows - just the one `hide_cursor` was called on.
 - On Windows, cursor grabs used to get perpetually canceled when the grabbing window lost focus. Now, cursor grabs automatically get re-initialized when the window regains focus and the mouse moves over the client area.
+- On Windows, only vertical mouse wheel events were handled. Now, horizontal mouse wheel events are also handled.
 
 # Version 0.18.1 (2018-12-30)
 

src/platform_impl/windows/event_loop.rs

@@ -994,6 +994,22 @@ unsafe extern "system" fn public_window_callback<T>(
             0
         },
 
+        winuser::WM_MOUSEHWHEEL => {
+            use event::MouseScrollDelta::LineDelta;
+            use event::TouchPhase;
+
+            let value = (wparam >> 16) as i16;
+            let value = value as i32;
+            let value = value as f32 / winuser::WHEEL_DELTA as f32;
+
+            subclass_input.send_event(Event::WindowEvent {
+                window_id: RootWindowId(WindowId(window)),
+                event: WindowEvent::MouseWheel { device_id: DEVICE_ID, delta: LineDelta(value, 0.0), phase: TouchPhase::Moved, modifiers: event::get_key_mods() },
+            });
+
+            0
+        },
+
         winuser::WM_KEYDOWN | winuser::WM_SYSKEYDOWN => {
             use event::ElementState::Pressed;
             use event::VirtualKeyCode;