Selection rules are not firing immediately in Quote Line Editor in Salesforce CPQ

I would say that

a) you might want to consider having quantity for fabric be measured in number of meters so you can have an automatic calculation based off of list price

b) It's not clear where your values of 1.2 and 1.4 are coming from. (I assume they are dynamically pulled from somewhere since you're not just increasing by 1.68?) If this is just a markup, you might want to populate the discount with a negative percentage

c) What you are probably looking for is a Price Rule.

For your use case, you'd probably do something like:

1. Check for that variable in the error condition of a price rule
2. If it exists, update the fields you want with a price action and a formula (you can try creating the formula in a formula field first to validate the syntax)

Here's an example of what your rule might look like

If you find that what you're trying to do is too complicated for a price rule, you can always use a Quote Calculator Plugin

Thanks to the Salesforce support I have the answer now.

CPQ only expects either all quote lines to be grouped or none. If the lines are grouped, SBQQ__Quote__c.SBQQ__LineItemsGrouped__c needs to be set to true, otherwise the Quote Lines editor will hang with the error I cited.