Fix surrogate pair handling on Windows (#1165)

* Fix surrogate pair handling on Windows

* Change high_surrogate to Option<u16>

* Format

CHANGELOG.md

@@ -8,7 +8,8 @@
 - On Windows, add touch pressure information for touch events.
 - On macOS, differentiate between `CursorIcon::Grab` and `CursorIcon::Grabbing`.
 - On Wayland, fix event processing sometimes stalling when using OpenGL with vsync.
-- Officially remove the Emscripten backend
+- Officially remove the Emscripten backend.
+- On Windows, fix handling of surrogate pairs when dispatching `ReceivedCharacter`.
 - On macOS 10.15, fix freeze upon exiting exclusive fullscreen mode.
 
 # 0.20.0 Alpha 3 (2019-08-14)

src/platform_impl/windows/event_loop.rs

@@ -1031,11 +1031,29 @@ unsafe extern "system" fn public_window_callback<T>(
 
         winuser::WM_CHAR => {
             use crate::event::WindowEvent::ReceivedCharacter;
+            let high_surrogate = 0xD800 <= wparam && wparam <= 0xDBFF;
+            let low_surrogate = 0xDC00 <= wparam && wparam <= 0xDFFF;
+
+            if high_surrogate {
+                subclass_input.window_state.lock().high_surrogate = Some(wparam as u16);
+            } else if low_surrogate {
+                let mut window_state = subclass_input.window_state.lock();
+                if let Some(high_surrogate) = window_state.high_surrogate.take() {
+                    let pair = [high_surrogate, wparam as u16];
+                    if let Some(Ok(chr)) = std::char::decode_utf16(pair.iter().copied()).next() {
+                        subclass_input.send_event(Event::WindowEvent {
+                            window_id: RootWindowId(WindowId(window)),
+                            event: ReceivedCharacter(chr),
+                        });
+                    }
+                }
+            } else {
                 let chr: char = mem::transmute(wparam as u32);
                 subclass_input.send_event(Event::WindowEvent {
                     window_id: RootWindowId(WindowId(window)),
                     event: ReceivedCharacter(chr),
                 });
+            }
             0
         }
 

src/platform_impl/windows/window_state.rs

@@ -32,6 +32,7 @@ pub struct WindowState {
     /// Used to supress duplicate redraw attempts when calling `request_redraw` multiple
     /// times in `EventsCleared`.
     pub queued_out_of_band_redraw: bool,
+    pub high_surrogate: Option<u16>,
     window_flags: WindowFlags,
 }
 
@@ -114,6 +115,7 @@ impl WindowState {
 
             fullscreen: None,
             queued_out_of_band_redraw: false,
+            high_surrogate: None,
             window_flags: WindowFlags::empty(),
         }
     }