The Win95/Win98 Caret Bug

Figure 1: Internal Windows function to optimize caret redrawing

BOOL SkipDrawErase(PWND pWnd, PCARETINFO pCaret)
{
    HRGN    hRgn;
    int     iResult;

    if (pWnd->hrgnUpdate == NULL)   // empty update region
        return(FALSE);

    if (pWnd->hrgnUpdate == 1)      // full update region
        return(TRUE);

    hRgn = CreateRectRgn(pCaret->x, pCaret->y,
                    pCaret->x + pCaret->width,
                    pCaret->y + pCaret->height);

    iResult = CombineRgn(hRgn, hRgn, pWnd->hrgnUpdate, RGN_AND);
    DeleteObject(hRgn);

    if (iResult > NULLREGION)
        return(TRUE);

    return(FALSE);
}