I just did an experiment. I saw somewhere on one of the W3.CSS pages that you could download the css file and run it locally.
I examined the file and saw at the top two statements with html definitions. The first one included box-sizing:border-box. When I looked up the box-sizing property, the default was content-box, as you stated.
I commented out the two html statements and then added a link to that local file before the link to my css file. Amazingly my left, center, and right div blocks displayed as expected again. I did have to add "line-height: normal" to my p, th, td, and a few other styles to eliminate the unwanted extra line height that was coming from the W3.css file.
With those problems fixed, I was able to re-implement the w3-tooltip element and create popup panels whose size and position could be individually controlled within a <span style=" "> tag block.
It is counterproductive for W3schools to offer a css framework that requires the user to waste so much time figuring out workarounds for unwanted behaviors. it spoils one's enthusiasm for the whole thing.