comctl32/header: Add support for HDS_FLAT style.

dlls/comctl32/header.c

@@ -330,6 +330,8 @@ HEADER_DrawItem (HEADER_INFO *infoPtr, HDC hdc, INT iItem, BOOL bHotTrack, LRESU
     else {
         HBRUSH hbr;
 
+        if (!(infoPtr->dwStyle & HDS_FLAT))
+        {
             if (infoPtr->dwStyle & HDS_BUTTONS) {
                 if (phdi->bDown) {
                     DrawEdge (hdc, &r, BDR_RAISEDOUTER,
@@ -341,6 +343,7 @@ HEADER_DrawItem (HEADER_INFO *infoPtr, HDC hdc, INT iItem, BOOL bHotTrack, LRESU
             }
             else
                 DrawEdge (hdc, &r, EDGE_ETCHED, BF_BOTTOM | BF_RIGHT | BF_ADJUST);
+        }
 
         hbr = CreateSolidBrush(GetBkColor(hdc));
         FillRect(hdc, &r, hbr);
@@ -547,7 +550,12 @@ HEADER_Refresh (HEADER_INFO *infoPtr, HDC hdc)
         if (theme != NULL) {
             DrawThemeBackground(theme, hdc, HP_HEADERITEM, HIS_NORMAL, &rcRest, NULL);
         }
-        else {
+        else if (infoPtr->dwStyle & HDS_FLAT) {
+            hbrBk = GetSysColorBrush(COLOR_3DFACE);
+            FillRect(hdc, &rcRest, hbrBk);
+        }
+        else
+        {
             if (infoPtr->dwStyle & HDS_BUTTONS)
                 DrawEdge (hdc, &rcRest, EDGE_RAISED, BF_TOP|BF_LEFT|BF_BOTTOM|BF_SOFT|BF_MIDDLE);
             else