Is Tracy complicated enough? Because it's imgui.

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.

	▲	well_ackshually 42 minutes ago \| parent [-]
		Absolutely zero difficulty redoing this in a react style renderer. The only complexity is being careful with your data dependencies so as to not needlessly rerender. Each pane is easily isolated, can share data with a view model scoped properly, etc. Writing it in an imperative toolkit is a "oops I forgot to update my data here" kind of hell. Data binding makes it slightly less worse