When does inspiration become theft?

When is it taking inspiration from a competitor, and when is it theft? I'd like to know the general questions to ask myself when reviewing competing products in creation of my own.

(I'd be surprised if something similar wasn't already asked, but a search revealed nothing)

For Example: Workflowy is an amazing tool, but it lacks a good deal of features I'd like to see. I'm making my own productivity tool and I'd like to include a number of main features in Workflowy in the new design (e.g. the 'zooming' into list items, hiding and showing sublists).

Part way through though, I started thinking that I may very well be stealing the very thing that makes Workflowy a great tool. On the other hand, I think that ignoring good innovations like these would hurt a developer's attempt at making the best tool in that category. Future users may think things like "Program B is wonderful, but it's a shame that it doesn't have this feature that Program A has".

Idea is not an IP. You can take anyone's idea and execute it.

Now what you CAN'T do is infringe on someone's patent.

You can check if your competitors have patents, but even if they do (which is highly unlikely) you can change one little piece of what's written in the 'claims' part of the patent and they will have a very hard time trying to enforce this patent.

I don't think you should feel shameful when borrowing inspirations from others, as long as it is legal. Few ones could start from total scratch, you always need some foundations. Nicer products, lower prices, better innovations, that's how free competition works.

For your concern "I think that ignoring good innovations like these would hurt a developer's attempt at making the best tool in that category". My opinion is, if no external stimulation like competition, less developers/product owners has incentive to improve their products.