From cd6eb1a6e5f3ce4d551d9cb2167f66d654a1137b Mon Sep 17 00:00:00 2001
From: Ioannis Tsakpinis <iotsakp@gmail.com>
Date: Wed, 1 Nov 2017 16:38:20 +0200
Subject: [PATCH] Win32: Improve window DPI change behavior

When a window transitions between monitors with different scale/dpi
settings:

1. It's automatically resized and repositioned by handling the
   WM_DPICHANGED event (Win8.1)
2. Its non-client area is automatically updated to match the new DPI
   (Win10)
---
 src/win32_init.c     |  2 ++
 src/win32_platform.h |  3 +++
 src/win32_window.c   | 23 +++++++++++++++++++++++
 3 files changed, 28 insertions(+)

diff --git a/src/win32_init.c b/src/win32_init.c
index ee7ccfdaa..737e113d9 100644
--- a/src/win32_init.c
+++ b/src/win32_init.c
@@ -98,6 +98,8 @@ static GLFWbool loadLibraries(void)
 
     _glfw.win32.user32.SetProcessDPIAware_ = (PFN_SetProcessDPIAware)
         GetProcAddress(_glfw.win32.user32.instance, "SetProcessDPIAware");
+    _glfw.win32.user32.EnableNonClientDpiScaling_ = (PFN_EnableNonClientDpiScaling)
+        GetProcAddress(_glfw.win32.user32.instance, "EnableNonClientDpiScaling");
     _glfw.win32.user32.ChangeWindowMessageFilterEx_ = (PFN_ChangeWindowMessageFilterEx)
         GetProcAddress(_glfw.win32.user32.instance, "ChangeWindowMessageFilterEx");
 
diff --git a/src/win32_platform.h b/src/win32_platform.h
index 1bfc638f4..5ac27efae 100644
--- a/src/win32_platform.h
+++ b/src/win32_platform.h
@@ -209,8 +209,10 @@ typedef HRESULT (WINAPI * PFN_DirectInput8Create)(HINSTANCE,DWORD,REFIID,LPVOID*
 
 // user32.dll function pointer typedefs
 typedef BOOL (WINAPI * PFN_SetProcessDPIAware)(void);
+typedef BOOL (WINAPI * PFN_EnableNonClientDpiScaling)(HWND);
 typedef BOOL (WINAPI * PFN_ChangeWindowMessageFilterEx)(HWND,UINT,DWORD,PCHANGEFILTERSTRUCT);
 #define SetProcessDPIAware _glfw.win32.user32.SetProcessDPIAware_
+#define EnableNonClientDpiScaling _glfw.win32.user32.EnableNonClientDpiScaling_
 #define ChangeWindowMessageFilterEx _glfw.win32.user32.ChangeWindowMessageFilterEx_
 
 // dwmapi.dll function pointer typedefs
@@ -322,6 +324,7 @@ typedef struct _GLFWlibraryWin32
     struct {
         HINSTANCE                       instance;
         PFN_SetProcessDPIAware          SetProcessDPIAware_;
+        PFN_EnableNonClientDpiScaling   EnableNonClientDpiScaling_;
         PFN_ChangeWindowMessageFilterEx ChangeWindowMessageFilterEx_;
     } user32;
 
diff --git a/src/win32_window.c b/src/win32_window.c
index 6d3e76cc3..06fbb5ed6 100644
--- a/src/win32_window.c
+++ b/src/win32_window.c
@@ -537,6 +537,16 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
 
                 break;
             }
+
+            case WM_NCCREATE:
+            {
+                // Windows 10: enable automatic non-client area scaling
+                if (EnableNonClientDpiScaling)
+                {
+                    EnableNonClientDpiScaling(hWnd);
+                }
+                break;
+            }
         }
 
         return DefWindowProcW(hWnd, uMsg, wParam, lParam);
@@ -988,6 +998,19 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
             return 0;
         }
 
+        case WM_DPICHANGED:
+        {
+            // Resize and reposition the window to match the new DPI
+            RECT* const suggestedRect = (RECT*)lParam;
+            SetWindowPos(hWnd, NULL,
+                         suggestedRect->left,
+                         suggestedRect->top,
+                         suggestedRect->right - suggestedRect->left,
+                         suggestedRect->bottom - suggestedRect->top,
+                         SWP_NOZORDER | SWP_NOACTIVATE);
+            return 0;
+        }
+
         case WM_SETCURSOR:
         {
             if (LOWORD(lParam) == HTCLIENT)