Only call SyncUpdateDC in EndPaint

src/gui/window.c

@@ -5748,6 +5748,7 @@ HDC GUIAPI BeginPaint (HWND hWnd)
 
 void GUIAPI EndPaint (HWND hWnd, HDC hdc)
 {
+    BOOL synced = FALSE;
     PMAINWIN pWin;
 
     MG_CHECK (MG_IS_NORMAL_WINDOW(hWnd));
@@ -5761,6 +5762,8 @@ void GUIAPI EndPaint (HWND hWnd, HDC hdc)
             __mg_update_secondary_dc (pWin, hdc, real_dc,
                     &pWin->pMainWin->update_rc, HT_CLIENT);
             ReleaseDC (real_dc);
+            SyncUpdateDC (real_dc);
+            synced = TRUE;
         }
     }
 
@@ -5790,6 +5793,9 @@ void GUIAPI EndPaint (HWND hWnd, HDC hdc)
         ShowCaretEx (hWnd, FALSE);
     }
 #endif
+
+    if (!synced)
+        SyncUpdateDC (hdc);
 }
 
 BOOL RegisterWindowClass (PWNDCLASS pWndClass)

src/include/dc.h

@@ -157,10 +157,11 @@ typedef BOOL (* CB_BITMAP_SCALER_FUNC )(
 /* Device Context */
 struct tagDC
 {
-    short DataType;  /* the data type, always be TYPE_HDC */
-    short DCType;    /* the dc type */
+    unsigned char DataType;  /* the data type, always be TYPE_HDC */
+    unsigned char DCType;    /* the dc type */
+    unsigned char inuse;
+    unsigned char bIsClient;
 
-    BOOL inuse;
     HWND hwnd;
 
     /* surface of this DC */
@@ -258,7 +259,6 @@ struct tagDC
     CLIPRGN  ecrgn;
 
     /* device rect */
-    BOOL bIsClient;
     RECT DevRC;
 
 #ifndef _MGSCHEMA_COMPOSITING
@@ -697,9 +697,8 @@ static inline void release_valid_dc (PMAINWIN pWin, HDC hdc)
             ReleaseSecondarySubDC (hdc);
     }
     else {
-        if (pWin->privCDC != hdc) {
+        if (pWin->privCDC != hdc)
             ReleaseDC (hdc);
-        }
     }
 }
 

src/newgdi/gdi.c

@@ -2289,14 +2289,13 @@ void GUIAPI ReleaseDC (HDC hDC)
 
     pdc = dc_HDC2PDC(hDC);
 
-    if (pdc->alpha_pixel_format) {
-        GAL_FreeFormat (pdc->alpha_pixel_format);
-        pdc->alpha_pixel_format = NULL;
+    /* Call SyncUpdateDC at the end of EndPaint to elimiate flickers.
+    if (pdc->bIsClient) {
+        LOCK (&__mg_gdilock);
+        GAL_SyncUpdate (pdc->surface);
+        UNLOCK (&__mg_gdilock);
     }
-
-    LOCK (&__mg_gdilock);
-    GAL_SyncUpdate (pdc->surface);
-    UNLOCK (&__mg_gdilock);
+    */
 
     pWin = (PMAINWIN)(pdc->hwnd);
     if (pWin && pWin->privCDC == hDC) {
@@ -2375,6 +2374,12 @@ void GUIAPI ReleaseDC (HDC hDC)
 #endif /* not defined _MGSCHEMA_COMPOSITING */
     }
     else {
+        /* since 5.0.0, only free pixel format for dc from slots */
+        if (pdc->alpha_pixel_format) {
+            GAL_FreeFormat (pdc->alpha_pixel_format);
+            pdc->alpha_pixel_format = NULL;
+        }
+
         EmptyClipRgn (&pdc->lcrgn);
         MAKE_REGION_INFINITE(&pdc->lcrgn);
         EmptyClipRgn (&pdc->ecrgn);