Clipping region for OnEraseBkgnd differs from clipping region for OnPaint - mfc

Question

I'm writing an mfc application.
I've a simple CWnd with OnEraseBkgnd and OnPaint. I'm experiencing some problems when another window covers partially my window.
So When the covering window is being moved out my CWnd gets WM_ERASEBKGND. I'm cleaning up dirty area and I return TRUE. What I can see here is that CDC I get has clipping box set and I use it so only a covered part is being erased. That's good.
But then WM_PAINT comes. CDC I get with GetDC does not have any clipping box so the whole window area is being repainted. This is a problem because in my paint event I use CDC::DrawText with a transparent background (CDC::SetBkMode(TRANSPARENT)) and painting the same text in the same not-erased place causes that text becomes 'bold'. Simply painting text over and over in the same place without wiping out background makes it look ugly.
Is it a normal behavior? Is my approach ok?
EDIT:
Here I attach more inforamtion about issue.
SSCCE:
class Foo : public CFrameWnd
{
public:
BOOL OnEraseBkgnd(CDC* pDC)
{
CRect rect;
pDC->GetClipBox(rect);
HBRUSH brush = ::GetSysColorBrush(COLOR_WINDOW);
HGDIOBJ pOld = pDC->SelectObject(brush);
const BOOL result = pDC->PatBlt(rect.left, rect.top, rect.Width(), rect.Height(), PATCOPY);
pDC->SelectObject(pOld);
return result;
}
void OnPaint()
{
CWnd::OnPaint();
CDC *dc = GetDC();
CRect clipBox;
dc->GetClipBox(clipBox);
CRect rect;
GetClientRect(rect);
CFont *font = &globalFont; // in my app here is the font I use but it doesn't matter
HFONT hFont = static_cast<HFONT>(font->GetSafeHandle());
auto oldFont = dc->SelectObject(hFont);
const int bkMode = dc->SetBkMode(TRANSPARENT);
dc->DrawText("AAAAAAAAA", -1, rect, 0);
dc->SetBkMode(bkMode);
dc->SelectObject(oldFont);
}
DECLARE_MESSAGE_MAP()
};
Creation:
Foo* f = new Foo;
f->Create( 0, "test", WS_VISIBLE| WS_OVERLAPPEDWINDOW);
Below how does the window look normally:
And below after moving the window so half of the text was out of monitor and then moved back:
So the part of window which was invisible was erased and then text was placed again. For the visible part of the window text was not erased and in OnPaint was redrawn again causing 'bold'.

Answer 1

You should be using CPaintDC, not just because it controls resources, as Barmak pointed out, but also because it retrieves clipping data. GetDC does not do that. (Barmak also mentioned PAINTSTRUCT, but it may not be clear that is the key to the clipping issue.)

Answer 2

This is unrelated issue, but GetDC is causing GDI resource leak in above code. ReleaseDC must be called before exiting the function:
void OnPaint()
{
CWnd::OnPaint();
CDC *dc = GetDC();
dc.DrawText(...);
...
ReleaseDC(dc);
}
Better yet, MFC has automatic cleanup with CClientDC
void myWnd::foo()
{
CClientDC dc(this);
dc.DrawText(...);
}
OnPaint can use special CPaintDC class which corresponds to PAINTSTRUCT:
void myWnd::OnPaint()
{
CPaintDC dc(this); //don't call CWnd::OnPaint
dc.DrawText(...);
}
Back to the problem:
It looks like part of background is not repainted, but part of the text is repainted. This makes it look ugly specially with clear type fonts (it looks like bold but it isn't).
You can fix the problem with this:
dc.SetBkMode(OPAQUE);
dc.SetBkColor(GetSysColor(COLOR_WINDOW));
dc.DrawText(L"AAAAAAAAA", -1, rect, 0);
Another option: override OnEraseBkgnd and force it to do nothing:
BOOL OnEraseBkgnd(CDC*)
{
return TRUE;
}
Do all of the painting in OnPaint()
void myWnd::OnPaint()
{
CPaintDC dc(this);
CRect rect;
GetClientRect(rect);
dc.FillSolidRect(rect, ::GetSysColor(COLOR_WINDOW) );
CFont *font = &globalFont;
auto oldFont = dc.SelectObject(font->GetSafeHandle());
dc.SetBkMode(TRANSPARENT);
dc.DrawText(L"AAAAAAAAA", -1, rect, 0);
dc.SelectObject(oldFont);
}