A user just deleted a file. Which feedback is right?

This one has no single winner - and that is the lesson. The right pattern depends entirely on what “delete” actually does. If the file drops into a trash you can restore from, the undo toast is the kinder choice: the action happens instantly, your flow is never blocked, and the way back sits right there for a few seconds.

If the delete is permanent and unrecoverable, that same convenience becomes a hazard - so a modal that stops you and makes you confirm earns its interruption. The discriminator is reversibility and stakes, not which pattern is “nicer”. See them side by side for the full contrast.

The trap is treating either pattern as a default. A modal on every trivial action trains people to click “confirm” without reading; an undo toast on something you cannot undo is a quiet way to lose someone's work. Read the stakes first, then choose.