The right way to use WM_TABSTOP

WM_TABSTOP works if you handle the messages in a dialog handler.
But, then you can't use tabs as characters in multiline edit boxes.

Handling the messages through IsDialogMessage() seems kinda hackerish. Is there any way to do it correctly?

The correct way is to use IsDialogMessage to get default keyboard handling as for dialogs and to subclass the edit control, handle its WM_GETDLGCODE message and return DLGC_WANTTAB from that handler.

See also msdn: How To Use the WM_GETDLGCODE Message.