Allow DPI to change on WM_DISPLAYCHANGED

Update the 'monitorResolution' global on WM_DISPLAYCHANGED, unless a
fixed DPI was specified by '-dpi' option or in .XWinrc file.

Handle WM_GETDPISCALEDSIZE to avoid DPI-scaling of our Windows when
moving between monitors with different DPI values.

hw/xwin/InitOutput.c

@@ -938,6 +938,8 @@ InitOutput(ScreenInfo * pScreenInfo, int argc, char *argv[])
     winConfigFiles();
 #endif
 
+    winUpdateDpi();
+
     /* Load preferences from XWinrc file */
     LoadPreferences();
 

hw/xwin/XWin.exe.manifest

@@ -15,7 +15,10 @@
 	</dependency>
 	<asmv3:application xmlns:asmv3="urn:schemas-microsoft-com:asm.v3">
 	   <asmv3:windowsSettings xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">
-	     <dpiAware>true</dpiAware>
+	     <dpiAware>true/pm</dpiAware>
+	   </asmv3:windowsSettings>
+	   <asmv3:windowsSettings xmlns="http://schemas.microsoft.com/SMI/2016/WindowsSettings">
+	     <dpiAwareness>PerMonitorV2,PerMonitor</dpiAwareness>
 	   </asmv3:windowsSettings>
 	</asmv3:application>
     <compatibility xmlns="urn:schemas-microsoft-com:compatibility.v1">

hw/xwin/win.h

@@ -57,7 +57,6 @@
 #define WIN_DEFAULT_BLACKPIXEL			0
 #define WIN_DEFAULT_LINEBIAS			0
 #define WIN_DEFAULT_E3B_TIME			50      /* milliseconds */
-#define WIN_DEFAULT_DPI				96
 #define WIN_DEFAULT_REFRESH			0
 #define WIN_DEFAULT_WIN_KILL			TRUE
 #define WIN_DEFAULT_UNIX_KILL			FALSE
@@ -1032,6 +1031,8 @@ winDoRandRScreenSetSize(ScreenPtr pScreen,
                         CARD16 width,
                         CARD16 height, CARD32 mmWidth, CARD32 mmHeight);
 
+void winUpdateDpi(void);
+
 /*
  * winmsgwindow.c
  */

hw/xwin/winglobals.c

@@ -80,6 +80,7 @@ Bool g_fSilentDupError = FALSE;
 Bool g_fNativeGl = TRUE;
 Bool g_fHostInTitle = TRUE;
 pthread_mutex_t g_pmTerminating = PTHREAD_MUTEX_INITIALIZER;
+Bool g_fixedDPI = FALSE;
 
 /*
  * Wrapped DIX functions

hw/xwin/winglobals.h

@@ -82,4 +82,6 @@ extern Bool g_fButton[3];
 
 extern pthread_mutex_t g_pmTerminating;
 
+extern Bool g_fixedDPI;
+
 #endif                          /* WINGLOBALS_H */

hw/xwin/winmultiwindowwndproc.c

@@ -48,6 +48,10 @@
 #define WM_DWMCOMPOSITIONCHANGED 0x031e
 #endif
 
+#ifndef WM_GETDPISCALEDSIZE
+#define WM_GETDPISCALEDSIZE 0x02E4
+#endif
+
 extern void winUpdateWindowPosition(HWND hWnd, HWND * zstyle);
 
 /*
@@ -1262,13 +1266,18 @@ winTopLevelWindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
         }
         break;
 
-
     case WM_DWMCOMPOSITIONCHANGED:
         /* This message is only sent on Vista/W7 */
         CheckForAlpha(hwnd, pWin, s_pScreenInfo);
 
         return 0;
 
+    case WM_GETDPISCALEDSIZE:
+        winDebug("winTopLevelWindowProc - WM_GETDPISCALEDSIZE\n");
+        // We don't change the lParam SIZE, so the Window retains the same size
+        // in pixels (rather than getting linearly scaled by the dpi value)
+        return TRUE;
+
     case WM_ASYNCMOVE:
         winAdjustWindowsWindow(pWin, hwnd);
         break;

hw/xwin/winprefsyacc.y

@@ -318,7 +318,10 @@ static void
 SetDPI (char *dpi)
 {
   if (!g_cmdline.customDPI)
-    monitorResolution = atoi (dpi);
+      {
+          monitorResolution = atoi (dpi);
+          g_fixedDPI = TRUE;
+      }
 }
 
 static void

hw/xwin/winprocarg.c

@@ -86,29 +86,6 @@ winInitializeScreenDefaults(void)
                   "winInitializeScreenDefaults - primary monitor w %d h %d\n",
                   (int) dwWidth, (int) dwHeight);
 
-    /* Set a default DPI, if no '-dpi' option was used */
-    if (monitorResolution == 0) {
-        HDC hdc = GetDC(NULL);
-
-        if (hdc) {
-            int dpiX = GetDeviceCaps(hdc, LOGPIXELSX);
-            int dpiY = GetDeviceCaps(hdc, LOGPIXELSY);
-
-            winErrorFVerb(2,
-                          "winInitializeScreenDefaults - native DPI x %d y %d\n",
-                          dpiX, dpiY);
-
-            monitorResolution = dpiY;
-            ReleaseDC(NULL, hdc);
-        }
-        else {
-            winErrorFVerb(1,
-                          "winInitializeScreenDefaults - Failed to retrieve native DPI, falling back to default of %d DPI\n",
-                          WIN_DEFAULT_DPI);
-            monitorResolution = WIN_DEFAULT_DPI;
-        }
-    }
-
     defaultScreenInfo.iMonitor = 1;
     defaultScreenInfo.hMonitor = MonitorFromWindow(NULL, MONITOR_DEFAULTTOPRIMARY);
     defaultScreenInfo.dwWidth = dwWidth;
@@ -942,6 +919,7 @@ ddxProcessArgument(int argc, char *argv[], int i)
      */
     if (IS_OPTION("-dpi")) {
         g_cmdline.customDPI = TRUE;
+        g_fixedDPI = TRUE;
         return 0;               /* Let DIX parse this again */
     }
 

hw/xwin/winrandr.c

@@ -302,3 +302,82 @@ winRandRInit(ScreenPtr pScreen)
 
     return TRUE;
 }
+
+/*
+  Update the DPI value
+
+ (or do nothing, if it's fixed)
+ */
+#define WIN_DEFAULT_DPI 96
+
+typedef enum MONITOR_DPI_TYPE {
+  MDT_EFFECTIVE_DPI = 0,
+} MONITOR_DPI_TYPE;
+
+typedef HRESULT WINAPI (*PFNGETDPIFORMONITOR)(HMONITOR, MONITOR_DPI_TYPE, UINT *, UINT *);
+
+void
+winUpdateDpi(void)
+{
+    /* If no '-dpi' option was used */
+    if (!g_fixedDPI)
+        {
+            // It seems that GetDeviceCaps reports the DPI when the Windows
+            // session started (or the process, if "Fix scaling for apps" is
+            // on), so instead use GetDpiForMonitor on the primary monitor, if
+            // that is available
+
+            static Bool tryLoad = TRUE;
+            if (tryLoad)
+                {
+                    LoadLibrary("shcore.dll");
+                    tryLoad = FALSE;
+                }
+
+            PFNGETDPIFORMONITOR pGDFM = (PFNGETDPIFORMONITOR) GetProcAddress(GetModuleHandle("shcore.dll"), "GetDpiForMonitor");
+
+            if (pGDFM)
+                {
+                    // Get HMONITOR of primary monitor
+                    // (by definition the primary monitor contains the point (0,0))
+                    const POINT ptZero = { 0, 0 };
+                    HMONITOR hPriMon = MonitorFromPoint(ptZero, MONITOR_DEFAULTTOPRIMARY);
+
+                    // Get DPI of primary monitor
+                    unsigned int dpiX, dpiY;
+                    HRESULT res = pGDFM(hPriMon, MDT_EFFECTIVE_DPI, &dpiX, &dpiY);
+                    if (res == S_OK)
+                        {
+                            ErrorF("winUpdateDpi - primary monitor native DPI x %u y %u\n", dpiX, dpiY);
+                            monitorResolution = dpiY;
+                        }
+                    else
+                        {
+                            ErrorF("winUpdateDpi - Failed to retrieve native DPI for primary monitor, falling back to default of %d DPI\n", WIN_DEFAULT_DPI);
+                            monitorResolution = WIN_DEFAULT_DPI;
+                        }
+                }
+            else
+                {
+                    HDC hdc = GetDC(NULL);
+
+                    if (hdc) {
+                        int dpiX = GetDeviceCaps(hdc, LOGPIXELSX);
+                        int dpiY = GetDeviceCaps(hdc, LOGPIXELSY);
+
+                        ErrorF("winUpdateDpi - native DPI x %d y %d\n", dpiX, dpiY);
+
+                        monitorResolution = dpiY;
+                        ReleaseDC(NULL, hdc);
+                    }
+                    else {
+                        ErrorF("winUpdateDpi - Failed to retrieve native DPI, falling back to default of %d DPI\n", WIN_DEFAULT_DPI);
+                        monitorResolution = WIN_DEFAULT_DPI;
+                    }
+                }
+        }
+    else
+        {
+            ErrorF("winUpdateDpi - Using fixed %d DPI\n", monitorResolution);
+        }
+}

hw/xwin/winwndproc.c

@@ -44,6 +44,10 @@
 #include "inputstr.h"
 #include "winclipboard/winclipboard.h"
 
+#ifndef WM_GETDPISCALEDSIZE
+#define WM_GETDPISCALEDSIZE 0x02E4
+#endif
+
 /*
  * Global variables
  */
@@ -140,6 +144,12 @@ winWindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
         }
         return 0;
 
+    case WM_GETDPISCALEDSIZE:
+        winDebug("winTopLevelWindowProc - WM_GETDPISCALEDSIZE\n");
+        // We don't change the lParam SIZE, so the Window retains the same size
+        // in pixels (rather than getting linearly scaled by the dpi value)
+        return TRUE;
+
     case WM_DISPLAYCHANGE:
         /*
            WM_DISPLAYCHANGE seems to be sent when the monitor layout or
@@ -253,6 +263,8 @@ winWindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
                         }
                 }
 
+                winUpdateDpi();
+
                 /*
                    XXX: probably a small bug here: we don't compute the work area
                    and allow for task bar
@@ -261,10 +273,8 @@ winWindowProc(HWND hwnd, UINT message, WPARAM wParam, LPARAM lParam)
                    the server is started
                  */
 
-                /* Set screen size to match new size, if it is different to current */
-                if (((dwWidth != 0) && (dwHeight != 0)) &&
-                    ((s_pScreenInfo->dwWidth != dwWidth) ||
-                     (s_pScreenInfo->dwHeight != dwHeight))) {
+                /* Set screen size to match new size */
+                if ((dwWidth != 0) && (dwHeight != 0)) {
                     winDoRandRScreenSetSize(s_pScreen,
                                             dwWidth,
                                             dwHeight,