Add API to influence indentation of expander in GtkTreeView
I'd might use GtkTreeView in Evolution to show list of messages, but users of the Evolution suggested a very interesting feature, to not indent children further to the right when the thread is "flat" (just single replies), which saves space and allows viewing more of the subject.
See the following screenshot, which shows two threads. The first thread is "flat", the second thread has only the third and the fourth messages shown as "flat":
I did not find a way to achieve this in the GtkTreeView, thus if it's not there, could you add API to be able to influence the indentation level for the expander by the widget users, please?
I can help with the implementation, just let me know what way it might be implemented the best.