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.