I figure I’d better document the solution(s) for others that may tread this path. A summary of some of the steps needed (of course built on top of Mark’s work!):
Ensure you register your modeless dialog with NPP (NPPM_MODELESSDIALOG, in Main.cs we call NppFormHelper.RegisterFormIfModeless(Form, bool)) The WPF System.Windows.Controls.UserControl should be in an System.Windows.Forms.Integration.ElementHost, which is in a System.Windows.Forms.Form. I’ve subclassed the ElementHost to handle WM_SHOWWINDOW to update the Visible property, and to trigger a (slightly delayed) refresh of the visuals when we get a WM_SIZE. There I invalidate the visuals and update layouts. There may be a way to skip this, but it’s working, so I’m not trying to fix it. In the WPF UserControl, when it’s Loaded, we have to get the presentation source (as a HwndSource) and add a hook to respond to the WM_GETDLGCODE and tell the source that we DLGC_WANTCHARS and DLGC_WANTARROWS, and maybe we also need DLGC_HASSETSEL.I’ve pulled most of it into a method called MakeModelessDialog. And now for a short recording of it working!
2025-06-28 Typing in Notepad++ modeless WPF dialog works.gif