How can I stop having white background after switching off scroll bars in CDialog?

Question

My dialog (in resource editor) has the resizing and maximize/minimize switched on.

But, if I detect that I don't need this functionality because their display is of a sufficient size I want to switch them off.

So I did this:

int CResizingDialog::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
    const CDC* pDC = GetDC();
    if (pDC != nullptr)
    {
        const int iHorz = pDC->GetDeviceCaps(HORZRES);
        const int iVert = pDC->GetDeviceCaps(VERTRES);

        bool showHorz = true;
        bool showVert = true;

        if (lpCreateStruct->cx < iHorz)
        {
            // Remove the WS_HSCROLL style to disable the horizontal scroll bar
            ModifyStyle(WS_HSCROLL, 0);
            showHorz = false;
        }

        if (lpCreateStruct->cy < iVert)
        {
            // Remove the WS_VSCROLL style to disable the vertical scroll bar
            ModifyStyle(WS_VSCROLL, 0);
            showVert = false;
        }

        if (!showHorz && !showVert && m_bModalWindow)
        {
            // Remove the WS_THICKFRAME style rtc to disable resizing
            ModifyStyle(WS_THICKFRAME | WS_MINIMIZEBOX | WS_MAXIMIZEBOX, 0);
        }
    }

    if (__super::OnCreate(lpCreateStruct) == -1)
        return -1;

    return 0;
}

It works, but look:

I now have two white rectangles on the right / bottom where I switched off the scroll bars. How do we address this?

Answer 1

I needed to make a call to SetWindowPos, like this:

int CResizingDialog::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
    const CDC* pDC = GetDC();
    if (pDC != nullptr)
    {
        const auto style = GetStyle();
        const int iHorz = pDC->GetDeviceCaps(HORZRES);
        const int iVert = pDC->GetDeviceCaps(VERTRES);

        bool showHorz{true};
        bool showVert{true};
        bool redrawWindow{false};

        bool hasHorzScrollBar{ false };
        if (style & WS_HSCROLL)
        {
            hasHorzScrollBar = true;
            // Do we need to switch off the horizontal scroll?
            if (lpCreateStruct->cx < iHorz)
            {
                // Remove the WS_HSCROLL style to disable the horizontal scroll bar
                ModifyStyle(WS_HSCROLL, 0);
                showHorz = false;
                redrawWindow = true;
            }
        }

        bool hasVertScrollBar{ false };
        if (style & WS_VSCROLL)
        {
            hasVertScrollBar = true;
            // Do we need to switch off the vertical scroll?
            if (lpCreateStruct->cy < iVert)
            {
                // Remove the WS_VSCROLL style to disable the vertical scroll bar
                ModifyStyle(WS_VSCROLL, 0);
                showVert = false;
                redrawWindow = true;
            }
        }

        if ((hasHorzScrollBar && !showHorz) &&
            (hasVertScrollBar && !showVert) && m_bModalWindow)
        {
            // Remove the WS_THICKFRAME style etc to disable resizing
            ModifyStyle(WS_THICKFRAME | WS_MINIMIZEBOX | WS_MAXIMIZEBOX, WS_DLGFRAME);
            redrawWindow = true;
        }

        // Apply new frame styles (important for cache update)
        if (redrawWindow)
        {
            // SM_CXSIZEFRAME thickness horizontal
            // SM_CYSIZEFRAME thickness vertical
            // SM_CXVSCROLL thickness vertical
            // SM_CYHSCROLL thickness horizontal

            int scrollVert{ 0 };
            if (hasVertScrollBar && !showVert)
            {
                scrollVert = GetSystemMetrics(SM_CXVSCROLL);
            }

            int scrollHorz{ 0 };
            if (hasHorzScrollBar && !showHorz)
            {
                scrollHorz = GetSystemMetrics(SM_CYHSCROLL);
            }

            SetWindowPos(nullptr,
                lpCreateStruct->x,
                lpCreateStruct->y,
                lpCreateStruct->cx - scrollVert,
                lpCreateStruct->cy - scrollHorz, SWP_NOMOVE | SWP_NOZORDER | SWP_FRAMECHANGED);
        }
    }

    if (__super::OnCreate(lpCreateStruct) == -1)
        return -1;

    return 0;
}