What problem does this solve or what need does it fill?

In css, we have calc() expression that can auto calculate and update when value is changed (e.g. window resized)

What solution would you like?

See bevyengine/bevy#5893

What alternative(s) have you considered?

Other solutions to solve and/or work around the problem presented.

Additional context

Any other information you would like to add such as related previous work,
screenshots, benchmarks, etc.