scaffolding . less loaded... why ?

I am having a page with a kartik-tab section.

When I click on a tab sometimes (!?) the entire css is screwed up because it suddenly uses scaffolding.less

Can someone please explain what this scaffolding.less is, and why/how it is loaded, so I can fix my problem

Any advise is welcome… I am totally lost at the moment

I found out what was going wrong… … the RenderAjax actually also REloaded the css files…