We have two GTK windows that we pop up as part of starting up a new session: tl-run-xstartup-feedback, and tl-select-profile. Both of these use the modern header bar in an effort to look like other GTK applications.
Unfortunately, this partially fails as we use Openbox as the window manager here, and it doesn't support everything GTK needs. Instead of getting the expected windows with rounded corners, we get GTK's fallback mode with square corners and an extra border around the entire window. This means we lose the familiarity we were after (and it looks ugly).
GTK requires three things¹ to enable the proper rendering:
* RGBA visuals
* WM support for _GTK_FRAME_EXTENTS
¹ See gtk_window_supports_client_shadow() in gtkwindow.c
Openbox supports none of this.
However, if I add a separate compositor, I should get the first two. And I should be able to fake the last one using xprop. But I tried this (with picom as the compositor), but GTK still refuses to enable things. So there is likely something more than xprop that is needed.