It might be a lot easier if we just add a helper function to get the selected room. That should avoid all the g_object_[gs]et_data stuff.

These should be named spaced, but we also try to avoid prototypes by just defining functions before they are needed.

you probably want gtk_widget_get_toplevel instead.

likewise on gtk_widget_get_toplevel

according to your g_action_group_add_entries call, this data should be set on dialog

according to the gtk4-demo menu progran, you just bind the child menu to the parent, and then call popup on it when you need it. Therefore avoiding needing to get an object from gtk builder and so on.

The menu element can and should be put directly in roomlist.ui at the same level as the template element. This will allow you to use the gtk_widget_class_bind_template_child.

delete-event was replaced with close-request. Which also means delete_win_cb should be renamed and have it's properties adjusted.

This should probably be visible.

the box itself wasn't set to fill or expand.

needs halign = fill or GTK_ALIGN_FILL as well.

seeds valign set to GTK_ALIGN_FILL

the old progress bar didn't have align or expand set.

action area got reworked see https://docs.gtk.org/gtk4/class.Dialog.html#gtkdialog-as-gtkbuildable for an example. And we should be using response ids instead of direct callbacks.

You can bind the selection via the ui file and gtk_widget_class_bind_template_child in pidgin_roomlist_class_init to avoid having to do this lookup. See https://docs.gtk.org/gtk4/class.TreeView.html#gtktreeview-as-gtkbuildable for the ui file syntax.

alignment is off, should be using spaces to align it with the first parameter after the (.

unused parameters should be marked with G_GNUC_UNUSED like G_GNUC_UNUSED GSimpleActions *action, G_GNUC_UNUSED GVariant *parameter

alignment again

d is unused and should be marked as so.

w is unused and should be marked as so.

There should probably be a if(PURPLE_IS_ROOMLIST_ROOM(room)) before this.

You don't need to typecast a gpointer, it'll be implicitly cast.

gdouble for consistency

no need for the cast

Typically we validate parameters before acting on them.
So this should use g_return_if_fail(PURPLE_IS_ACCOUNT(account)); before creating the dialog, which that that point should be valid and not need a g_return_if_* macro.

this should probably be start i think?

Action area has been removed in GTK4. The child buttons are just normal children with an additional attribute of type="action" on them.
https://docs.gtk.org/gtk4/class.Dialog.html#gtkdialog-as-gtkbuildable

We should probably make sure that a room was selected before attempting to join it.

Ship It!

Awesome work, thank you so much for seeing this through until the end!!