Win32: Fix non-BMP Unicode codepoint input

Supplementary Plane codepoints from WM_CHAR and WM_SYSCHAR messages were
reported as UTF-16 surrogate pairs.

Related to #1635.

(cherry picked from commit 6ce2070392)

README.md

@@ -120,6 +120,7 @@ information on what to include when reporting a bug.
 
  - Bugfix: Some extension loader headers did not prevent default OpenGL header
    inclusion (#1695)
+ - [Win32] Bugfix: Non-BMP Unicode codepoint input was reported as UTF-16
  - [Cocoa] Changed `EGLNativeWindowType` from `NSView` to `CALayer` (#1169)
  - [Cocoa] Bugfix: Non-BMP Unicode codepoint input was reported as UTF-16
    (#1635)

src/win32_platform.h

@@ -318,6 +318,8 @@ typedef struct _GLFWwindowWin32
 
     // The last received cursor position, regardless of source
     int                 lastCursorPosX, lastCursorPosY;
+    // The last recevied high surrogate when decoding pairs of UTF-16 messages
+    WCHAR               highSurrogate;
 
 } _GLFWwindowWin32;
 

src/win32_window.c

@@ -645,11 +645,35 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
 
         case WM_CHAR:
         case WM_SYSCHAR:
+        {
+            if (wParam >= 0xd800 && wParam <= 0xdbff)
+                window->win32.highSurrogate = (WCHAR) wParam;
+            else
+            {
+                unsigned int codepoint = 0;
+
+                if (wParam >= 0xdc00 && wParam <= 0xdfff)
+                {
+                    if (window->win32.highSurrogate)
+                    {
+                        codepoint += (window->win32.highSurrogate - 0xd800) << 10;
+                        codepoint += (WCHAR) wParam - 0xdc00;
+                        codepoint += 0x10000;
+                    }
+                }
+                else
+                    codepoint = (WCHAR) wParam;
+
+                window->win32.highSurrogate = 0;
+                _glfwInputChar(window, codepoint, getKeyMods(), uMsg != WM_SYSCHAR);
+            }
+
+            return 0;
+        }
+
         case WM_UNICHAR:
         {
-            const GLFWbool plain = (uMsg != WM_SYSCHAR);
+            if (wParam == UNICODE_NOCHAR)
-
-            if (uMsg == WM_UNICHAR && wParam == UNICODE_NOCHAR)
             {
                 // WM_UNICHAR is not sent by Windows, but is sent by some
                 // third-party input method engine
@@ -657,7 +681,7 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
                 return TRUE;
             }
 
-            _glfwInputChar(window, (unsigned int) wParam, getKeyMods(), plain);
+            _glfwInputChar(window, (unsigned int) wParam, getKeyMods(), GLFW_TRUE);
             return 0;
         }