CUSTOMDRAW in Win32 Header not appropriately drawing outside columns area - c++

Question

I am currently subclassing a ListView control to support custom color schemes - especially Dark Mode themes.
So first, I changed the ListView and associated Header with:
SetWindowTheme(hwndListView, isDarkModeEnabled() ? L"Explorer" : nullptr, nullptr);
SetWindowTheme(hwndListViewHeader, isDarkModeEnabled() ? L"ItemsView" : nullptr, nullptr);
I can set the ListView background color to any custom color I choose, while answering to the WM_THEMECHANGED message and doing a ListView_SetBkColor() macro.
Unfortunately, it seems I can't do it with the associated Header Control, I must resort to Custom Draw. So I did. I subclassed the ListView, because this is the window handle that will receive WM_NOTIFY messages from the Header control, and when responding to NM_CUSTOMDRAW message, my code looks like this:
case WM_NOTIFY:
{
LPNMHDR nmhdr = reinterpret_cast<LPNMHDR>(lParam);
if (nmhdr->code == NM_CUSTOMDRAW)
{
LPNMCUSTOMDRAW lpcd = reinterpret_cast<LPNMCUSTOMDRAW>(lParam);
switch (lpcd->dwDrawStage)
{
case CDDS_PREPAINT:
{
if (!PluginDarkMode::isEnabled())
return CDRF_DODEFAULT;
FillRect(lpcd->hdc, &lpcd->rc, getDarkerBackgroundBrush());
return CDRF_NOTIFYITEMDRAW | CDRF_NOTIFYPOSTPAINT;
}
case CDDS_ITEMPREPAINT:
{
return DrawListViewHeaderItem(lParam); // This function draws the item entirely and then returns CDRF_SKIPDEFAULT.
}
case CDDS_POSTPAINT:
// Calculates the undrawn border outside columns
HWND hHeader = lpcd->hdr.hwndFrom;
int count = static_cast<int>(Header_GetItemCount(hHeader));
int colsWidth = 0;
RECT wRc = {};
for (int i = 0; i < count; i++)
{
Header_GetItemRect(hHeader, i, &wRc);
colsWidth += wRc.right - wRc.left;
}
RECT clientRect;
GetClientRect(hHeader, &clientRect);
if (clientRect.right > (colsWidth + 1))
{
clientRect.left = colsWidth + 1;
HDC hdc = GetDC(hHeader);
FillRect(hdc, &clientRect, getDarkerBackgroundBrush());
ReleaseDC(hHeader, hdc);
}
return CDRF_SKIPDEFAULT;
}
}
break;
}
Now, my problem is that, although this seems fine, my control will always end up with a black rectangle outside the column's area (the empty space where no column exists). And this effect persists up until I move the mouse outside the header control area.
For example:
And when the mouse leaves the area:
Any ideas of how to solve this?