SplitPane size in px vs %

[stropitek]

See the difference between setting a size of 5% vs 30px in this example which has a SplitPane of 600px:

https://react-science.pages.dev/stories/?path=/story/components-splitpane--control&args=controlledSide:end

The difference is probably due to browser specifications but I'm unaware of what those are. I'm not suggesting to change this, just keen on having an explanation if anyone has one.

#700