Olilarkin reply from the Slack:
IGEditor::LayoutUI() default implementation calls IGEditor::mLayoutFunc which specified in the examples using a lambda in the plugin ctor (if you want you can override LayoutUI() instead of using lambda).
When you attach the corner resizer, 2nd arg chooses if the layout function is called when GUI is resized. pGraphics->AttachCornerResizer(EUIResizerMode::Scale, TRUE);
Near the top of the layout function you can test the incoming pGraphics->NControls() to see if controls already exist. If so, then you can update existing control positions and return from the layout func (important, otherwise you may re-add controls)
3:55
here is an example https://gist.github.com/olilarkin/d2e9a2f3675421064dfc7cd1990ed654