How to create a simple dynamically expanding rectangle geometry

Ok, one more question regarding the BufferGeometry:

Basically I’ve to create a geometry that looks like the following or? Is there a simple method to generate this circle geometries? I don’t want to implement that on my own… Seems like it’s possible to merge a plane geometry with a circle geometry but in that case, I’d lose the vertex indexes (red dots) which I need for my transformation or?

rounded-rect-wireframe

(sorry for the bad quality :see_no_evil:)