WM_NCCALCSIZE, custom client area, and scroll bars

Question

I have an MFC app that embeds a Scintilla text edit control. I want to customize the Scintilla control to display some custom controls next to the vertical scrollbar. Essentially, I want to render some controls in the orange area below, where the green area represent the scroll bars:

I tried overriding the WM_NCCALCSIZE message of the Scintilla window and subtracting an offset from the right side of the client rectangle. Here is the code:

void CScintillaCtrl::OnNcCalcSize(BOOL bCalcValidRects, NCCALCSIZE_PARAMS* lpncsp)
{
    CWnd::OnNcCalcSize(bCalcValidRects, lpncsp);
    lpncsp->rgrc[0].right -= 100;
}

However, this causes the vertical and horizontal scroll bars to reposition themselves to account for the smaller client width, as shown below:

I'm not sure if this behavior is caused by Scintilla or Windows. Is there a way I can adjust the client area and preserve the positions of the scroll bars?

Answer 1

I found a Scintilla specific solution. I can use the SCI_SETMARGINRIGHT command to add a margin to the right side of the client area, and then render my controls inside that.