Use SetWindowCompositionAttribute for alpha-awareness on W10

Using DwmEnableBlurBehindWindow() as we do works fine on Vista and Windows 7
but doesn't render properly on Windows 8.1, 10

Use the (undocumented) SetWindowCompositionAttribute function to get the
same appearance on Windows 10.

For Windows 8.1, we seem to be out of luck.

1 files changed, 47 insertions, 5 deletions

diff --git a/src/main.c b/src/main.c
index b50eaa4..58935d1 100644
--- a/src/main.c
+++ b/src/main.c
@@ -33,6 +33,29 @@
 #include "wndproc.h"
 #include "wincursor.h"
 
+// When mingw-w64-headers 5.0 is available, just include versionhelper.h for these
+static
+BOOL IsWindowsVersionOrGreater(WORD major, WORD minor, WORD servpack)
+{
+  OSVERSIONINFOEXW vi = {sizeof(vi),major,minor,0,0,{0},servpack,0,0,0,0};
+  return VerifyVersionInfoW(&vi, VER_MAJORVERSION|VER_MINORVERSION|VER_SERVICEPACKMAJOR,
+         VerSetConditionMask(VerSetConditionMask(VerSetConditionMask(0,
+            VER_MAJORVERSION,VER_GREATER_EQUAL),
+            VER_MINORVERSION,VER_GREATER_EQUAL),
+            VER_SERVICEPACKMAJOR, VER_GREATER_EQUAL));
+}
+
+static
+BOOL IsWindowsVistaOrGreater(void) {
+  return IsWindowsVersionOrGreater(HIBYTE(_WIN32_WINNT_VISTA), LOBYTE(_WIN32_WINNT_VISTA), 0);
+}
+
+static
+BOOL IsWindows8OrGreater(void) {
+  return IsWindowsVersionOrGreater(HIBYTE(_WIN32_WINNT_WIN8), LOBYTE(_WIN32_WINNT_WIN8), 0);
+}
+
+//
 #define WM_XCWM_CREATE  WM_USER
 #define WM_XCWM_DESTROY (WM_USER+1)
 #define WM_XCWM_CURSOR (WM_USER+2)
@@ -202,16 +225,34 @@ int main(int argc, char **argv)
   // initialize semaphore used for synchronization
   sem_init(&semaphore, 0, 0);
 
-  // Get access to DwmEnableBlurBehindWindow, if available, so we can take advantage of it
-  // on Vista and later, but still run on earlier versions of Windows
+  // The way to tell Windows that it should use the alpha channel from a GDI
+  // window in DWM desktop composition depends on the Windows version
   HMODULE hDwmApiLib = LoadLibraryEx("dwmapi.dll", NULL, 0);
-  if (hDwmApiLib)
-    pDwmEnableBlurBehindWindow = (PFNDWMENABLEBLURBEHINDWINDOW)GetProcAddress(hDwmApiLib, "DwmEnableBlurBehindWindow");
-  DEBUG("DwmEnableBlurBehindWindow %s\n", pDwmEnableBlurBehindWindow ? "found" : "not found");
+  HMODULE hUser32 = LoadLibraryEx("user32.dll", NULL, 0);
+
+  if (IsWindowsVistaOrGreater() && !IsWindows8OrGreater())
+    {
+      // Use DwmEnableBlurBehindWindow, if available, to turn on alpha channel
+      // use on Windows Vista and Windows 7
+
+      if (hDwmApiLib)
+        pDwmEnableBlurBehindWindow = (PFNDWMENABLEBLURBEHINDWINDOW)GetProcAddress(hDwmApiLib, "DwmEnableBlurBehindWindow");
+      DEBUG("DwmEnableBlurBehindWindow %s\n", pDwmEnableBlurBehindWindow ? "found" : "not found");
+    }
+  else if (IsWindows8OrGreater())
+    {
+      // Use SetWindowCompositionAttribute, if available, to turn on alpha
+      // channel use on Windows 8, 8.1, 10
+      if (hUser32)
+        pSetWindowCompositionAttribute = (PFNSETWINDOWCOMPOSITIONATTRIBUTE) GetProcAddress(hUser32, "SetWindowCompositionAttribute");
+      DEBUG("SetWindowCompositionAttribute %s\n", pSetWindowCompositionAttribute ? "found" : "not found");
+    }
+
   if (nodwm)
     {
       DEBUG("DWM disabled by --nodwm option\n");
       pDwmEnableBlurBehindWindow = NULL;
+      pSetWindowCompositionAttribute = NULL;
     }
 
   xcwm_context_flags_t flags = 0;
@@ -361,6 +402,7 @@ int main(int argc, char **argv)
   xcwm_context_close(context);
 
   FreeLibrary(hDwmApiLib);
+  FreeLibrary(hUser32);
 
   sem_destroy(&semaphore);
 }