Attempting to simulate client area based window dragging by returning HTCAPTION under a WM_NCHITTEST (excluding HTCLIENT & appropriate areas) works flawlessly when used with a parent window - however presence of child windows such as tabs placeholder windows, even when set to the extended style WS_EX_TRANSPARENT, cause clicks to fail to pass WM_NCHITTEST messages to the parent window (and attempting to process local WM_NCHITEST messages in a similar fashion produces the expected effect of dragging the child window around the parent rather than the parent itself).
Given that every area in that tab child window appears to be considered to be client area, processing WM_LBUTTONDOWN instead appears to produce the desired effect (see below):
case WM_LBUTTONDOWN: {
SendMessage(mainWnd.hWnd, WM_NCLBUTTONDOWN, HTCAPTION, lParam);
break;
}
Where mainWnd.hWnd is the parent window handle (hWnd is a member of a designed window properties helper class)
Although this produces the desired effect, I'm confused at whether WS_EX_TRANSPARENT is actually meant to allow clicks to pass through to underlying windows, and whether there is a more appropriate solution?