Win32: Fix rect of undecorated maximized windows

A window created maximized and undecorated would cover the whole monitor
Windows placed it on instead of just that monitor's workarea.

This commit adjusts the maximized rect to cover just the workarea,
similar to how undecorated windows that become maximized are handled
during WM_GETMINMAXINFO.

Fixes #1806

README.md

@@ -207,6 +207,8 @@ information on what to include when reporting a bug.
    disconnected (#1615)
  - [Win32] Bugfix: Key name update modified global key state on Windows 10 1607
    and later (#2018)
+ - [Win32] Bugfix: A window created maximized and undecorated would cover the whole
+   monitor (#1806)
  - [Cocoa] Added support for `VK_EXT_metal_surface` (#1619)
  - [Cocoa] Added locating the Vulkan loader at runtime in an application bundle
  - [Cocoa] Moved main menu creation to GLFW initialization time (#1649)

src/win32_window.c

@@ -1292,18 +1292,22 @@ static int createNativeWindow(_GLFWwindow* window,
     window->win32.scaleToMonitor = wndconfig->scaleToMonitor;
     window->win32.keymenu = wndconfig->win32.keymenu;
 
-    // Adjust window rect to account for DPI scaling of the window frame and
-    // (if enabled) DPI scaling of the content area
-    // This cannot be done until we know what monitor the window was placed on
     if (!window->monitor)
     {
         RECT rect = { 0, 0, wndconfig->width, wndconfig->height };
         WINDOWPLACEMENT wp = { sizeof(wp) };
+        const HMONITOR mh = MonitorFromWindow(window->win32.handle,
+                                              MONITOR_DEFAULTTONEAREST);
+
+        // Adjust window rect to account for DPI scaling of the window frame and
+        // (if enabled) DPI scaling of the content area
+        // This cannot be done until we know what monitor the window was placed on
+        // Only update the restored window rect as the window may be maximized
 
         if (wndconfig->scaleToMonitor)
         {
             float xscale, yscale;
-            _glfwGetWindowContentScaleWin32(window, &xscale, &yscale);
+            _glfwGetHMONITORContentScaleWin32(mh, &xscale, &yscale);
 
             if (xscale > 0.f && yscale > 0.f)
             {
@@ -1323,11 +1327,26 @@ static int createNativeWindow(_GLFWwindow* window,
         else
             AdjustWindowRectEx(&rect, style, FALSE, exStyle);
 
-        // Only update the restored window rect as the window may be maximized
         GetWindowPlacement(window->win32.handle, &wp);
         wp.rcNormalPosition = rect;
         wp.showCmd = SW_HIDE;
         SetWindowPlacement(window->win32.handle, &wp);
+
+        // Adjust rect of maximized undecorated window, because by default Windows will
+        // make such a window cover the whole monitor instead of its workarea
+
+        if (wndconfig->maximized && !wndconfig->decorated)
+        {
+            MONITORINFO mi = { sizeof(mi) };
+            GetMonitorInfo(mh, &mi);
+
+            SetWindowPos(window->win32.handle, HWND_TOP,
+                         mi.rcWork.left,
+                         mi.rcWork.top,
+                         mi.rcWork.right - mi.rcWork.left,
+                         mi.rcWork.bottom - mi.rcWork.top,
+                         SWP_NOACTIVATE | SWP_NOZORDER);
+        }
     }
 
     DragAcceptFiles(window->win32.handle, TRUE);