Handle horizontal wheel input (Windows) (#792)

* add handler for horizontal wheel input

* add changlelog message re now handling horiz scroll on windows

CHANGELOG.md

@@ -8,6 +8,7 @@
 - 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/windows/events_loop.rs

@@ -711,6 +711,22 @@ unsafe fn callback_inner(
             0
         },
 
+        winuser::WM_MOUSEHWHEEL => {
+            use events::MouseScrollDelta::LineDelta;
+            use events::TouchPhase;
+
+            let value = (wparam >> 16) as i16;
+            let value = value as i32;
+            let value = value as f32 / winuser::WHEEL_DELTA as f32;
+
+            send_event(Event::WindowEvent {
+                window_id: SuperWindowId(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 events::ElementState::Pressed;
             use events::VirtualKeyCode;