Moving (repositioning) a Child Window or Dialog in Gtk / Gtkmm

Question

A child Gtk::Window or Gtk::Dialog may be moved around by dragging on the title bar. Being top level windows this activity requires support from the window manager. What is the mechanism by which Gtk requests the window manager to move the position of the window?

Background

I have a Gtk application running on a custom Linux distribution (based on Yocto running Waland/Weston). The application is developed on Ubuntu 20 which has both X11 backend and Wayland backend. The child dialogs or windows that are spawned by the main window are perfectly centered on the main window (in Ubuntu on both backends). However on the target (with Weston) the dialogs or windows appear at random position. Now I understand that this is reported in several forums (like this one in stackoverflow itself).

Different Approach?

With what ever little I know I tried Gtk::Window::move, Gdk::Window::move and even dared to play with Wayland surfaces (gdk_wayland_window_set_transient_for_exported ) but with no avail. That left me wondering how the user is able to move such child windows by grabbing the header bar (or title bar as Gtk::Window calls it) even under Weston. If I get to know how this works then perhaps I can emulate a grab-drag to position the window where ever I want. I tried sifting through gtkwindow.c to find out what happens when one sets the title bar using the function gtk_window_set_titlebar but the rabbit hole went a little too deep.

It would be great if someone can point me in the right direction, at least quote some functions whose implementation I can study to get this working....

Answer 1

Your question consists of multiple smaller ones, so I'll try to give a shot at answering each and one of them.

The general idea is that Wayland is quite minimal, so to make it suitable for desktop use cases, you need a protocol extension. This extension is called XDG Shell.

A child Gtk::Window or Gtk::Dialog may be moved around by dragging on the title bar. Being top level windows this activity requires support from the window manager. What is the mechanism by which Gtk requests the window manager to move the position of the window?

This first part is described in the Wayland book, but the idea is that you forward an input event (usually a drag) back to the compositor, who will know what do with it. That might mean moving the window (or not moving it, if you've reached the edge of the screen.

However on the target (with Weston) the dialogs or windows appear at random position. Now I understand that this is reported in several forums (like this one in stackoverflow itself).

Note that your confusing 2 questions here: one is where to put a child window, compared to a parent window, while the second sentence here talks about position any toplevel window. There is also a section in the Wayland book on popups (part of XDG shell also) which also describe something similar.

So whether you can arbitrarily move windows: the answer is no.

The most important question then becomes: what can you do to solve your problems with Weston? It's hard to say without any kind of code. You might want to make sure you set the GtkDialog parent when constructing it (also known as the transient_for property. You might want to play around with the modal flag also. There might be other options too, but it's a bit of a blind guess.