[PD-dev] help figuring out how to know when in edit mode and setting cursor

Hi, I am hoping to learn how to better code with tcl/tk and GUI design and one of the things I would like to do is trying to make a PR so iemguis can be resized, like comments can.

I also wanna do that for my externals. But the mechanism I have there in an object like [else/scope~] and [else/note] is inherited from Cyclone and has a much different strategy, so it seems.

Could anyone help me figuring out Pd's code, where exactly to look and dive in, to understand how an object like comment is aware of when Pd gets into edit mode, so it changes its look to show a draggable sidebar. Also, how is it aware that when reaching that sidebar it must show a different double sided arrow cursor?

thanks