Upgrade tool (3 -> 4) migrates maxWidth configuration to container vars #19555

What version of Tailwind CSS are you using?

For example: v4.1.18

What build tool (or framework if it abstracts the build tool) are you using?

Vite ^7

What version of Node.js are you using?

v22.21.1

What browser are you using?

Firefox Nightly

What operating system are you using?

MacOS

Reproduction URL

If you use custom maxWidth values in your Tailwind v3 config and then migrate to Tailwind 4, those values are moved to the --container vars which then affect built-in classes like columns-3.

I made a reproduction repository:

1. Commit with Tailwind v3: infomiho/tailwind-upgrade-container-repro@385433c

3. Commit with Tailwind v4 after running the upgrade tool: infomiho/tailwind-upgrade-container-repro@9f1ef85

Describe your issue

I'd expect columns-3 not to be broken after an upgrade :) Not sure what's the way to pull this off.