That looks quite simple. Think about something like this, a commercial SystemVerilog simulator (this only shows a fraction of the UI).

https://blog.reds.ch/wp-content/uploads/2018/09/questa13.png

Or something like Visual Studio.

Obviously most GUIs are not nearly that complex so immediate mode can get you quite far. Its biggest limitation is that it makes it hard to do some layouts. Your GUI layout becomes dictated by your data dependencies which is quite awkward.