Esme Povirk (@madewokherd) commented about dlls/comdlg32/navbar.c:
PAINTSTRUCT ps;HDC hdc = BeginPaint(hwnd, &ps);RECT rc;HBRUSH old_brush;HBRUSH bg_brush = GetSysColorBrush(COLOR_BTNFACE);HBRUSH frame_brush = GetSysColorBrush(COLOR_3DSHADOW);GetClientRect(hwnd, &rc);/* clear background to avoid border trailing when resizing */FillRect(hdc, &rc, bg_brush);/* draw the 1px frame */FrameRect(hdc, &rc, frame_brush);/* erase the left border, retaining the top and bottom pixels */
I think you could remove the need for this by drawing the left border outside the window.