Add GLFW_SCALE_TO_MONITOR

This adds the GLFW_SCALE_TO_MONITOR window hint for automatically
resizing the content area of a window to the requested size times the
monitor content scale each time it is placed on a new monitor.  This
only applies to windowed mode windows and includes the initial placement
at window creation.

This hint only has an effect on platforms where screen coordinates and
pixels always map 1:1 such as Windows and X11.  Platforms like macOS
instead change the resolution of the framebuffer independently of the
window size.

Related to #676.
Related to #1115.

src/internal.h

@@ -268,6 +268,7 @@ struct _GLFWwndconfig
     GLFWbool      maximized;
     GLFWbool      centerCursor;
     GLFWbool      focusOnShow;
+    GLFWbool      scaleToMonitor;
     struct {
         GLFWbool  retina;
         char      frameName[256];

src/win32_platform.h

@@ -306,6 +306,7 @@ typedef struct _GLFWwindowWin32
     GLFWbool            maximized;
     // Whether to enable framebuffer transparency on DWM
     GLFWbool            transparent;
+    GLFWbool            scaleToMonitor;
 
     // The last received cursor position, regardless of source
     int                 lastCursorPosX, lastCursorPosY;

src/win32_window.c

@@ -1063,6 +1063,9 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
 
         case WM_GETDPISCALEDSIZE:
         {
+            if (window->win32.scaleToMonitor)
+                break;
+
             // Adjust the window size to keep the client area size constant
             if (_glfwIsWindows10CreatorsUpdateOrGreaterWin32())
             {
@@ -1230,15 +1233,34 @@ static int createNativeWindow(_GLFWwindow* window,
                                     WM_COPYGLOBALDATA, MSGFLT_ALLOW, NULL);
     }
 
-    // Adjust window size to account for the DPI scaled window frame
+    window->win32.scaleToMonitor = wndconfig->scaleToMonitor;
+
+    // Adjust window size to account for DPI scaling of the window frame and
+    // optionally DPI scaling of the client area
     // This cannot be done until we know what monitor it was placed on
-    if (_glfwIsWindows10AnniversaryUpdateOrGreaterWin32() && !window->monitor)
+    if (!window->monitor)
     {
         RECT rect = { 0, 0, wndconfig->width, wndconfig->height };
+
+        if (wndconfig->scaleToMonitor)
+        {
+            float xscale, yscale;
+            _glfwPlatformGetWindowContentScale(window, &xscale, &yscale);
+            rect.right = (int) (rect.right * xscale);
+            rect.bottom = (int) (rect.bottom * yscale);
+        }
+
         ClientToScreen(window->win32.handle, (POINT*) &rect.left);
         ClientToScreen(window->win32.handle, (POINT*) &rect.right);
-        AdjustWindowRectExForDpi(&rect, style, FALSE, exStyle,
-                                 GetDpiForWindow(window->win32.handle));
+
+        if (_glfwIsWindows10AnniversaryUpdateOrGreaterWin32())
+        {
+            AdjustWindowRectExForDpi(&rect, style, FALSE, exStyle,
+                                     GetDpiForWindow(window->win32.handle));
+        }
+        else
+            AdjustWindowRectEx(&rect, style, FALSE, exStyle);
+
         SetWindowPos(window->win32.handle, NULL,
                      rect.left, rect.top,
                      rect.right - rect.left, rect.bottom - rect.top,

src/window.c

@@ -369,6 +369,9 @@ GLFWAPI void glfwWindowHint(int hint, int value)
         case GLFW_COCOA_GRAPHICS_SWITCHING:
             _glfw.hints.context.nsgl.offline = value ? GLFW_TRUE : GLFW_FALSE;
             return;
+        case GLFW_SCALE_TO_MONITOR:
+            _glfw.hints.window.scaleToMonitor = value ? GLFW_TRUE : GLFW_FALSE;
+            return;
         case GLFW_CENTER_CURSOR:
             _glfw.hints.window.centerCursor = value ? GLFW_TRUE : GLFW_FALSE;
             return;

src/x11_window.c

@@ -592,6 +592,15 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
                                    const _GLFWwndconfig* wndconfig,
                                    Visual* visual, int depth)
 {
+    int width = wndconfig->width;
+    int height = wndconfig->height;
+
+    if (wndconfig->scaleToMonitor)
+    {
+        width *= _glfw.x11.contentScaleX;
+        height *= _glfw.x11.contentScaleY;
+    }
+
     // Create a colormap based on the visual used by the current context
     window->x11.colormap = XCreateColormap(_glfw.x11.display,
                                            _glfw.x11.root,
@@ -617,7 +626,7 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
         window->x11.handle = XCreateWindow(_glfw.x11.display,
                                            _glfw.x11.root,
                                            0, 0,
-                                           wndconfig->width, wndconfig->height,
+                                           width, height,
                                            0,      // Border width
                                            depth,  // Color depth
                                            InputOutput,
@@ -720,7 +729,7 @@ static GLFWbool createNativeWindow(_GLFWwindow* window,
         XFree(hints);
     }
 
-    updateNormalHints(window, wndconfig->width, wndconfig->height);
+    updateNormalHints(window, width, height);
 
     // Set ICCCM WM_CLASS property
     {