User prompt triggered by action in Perform Actions should cause the command to fail with 'unexpected alert open'

This is a straw man proposal to align user prompt handler usage with Execute Script. This was just changed so the user prompt handler is never invoked mid-command; instead the command fails with unexpected alert open' and a subsequent session command will run the user prompt handler.

It is kind of weird to expect to end an action chain with an unexpected alert open, in the sense that you'd always handle this case explicitly with an Alert-related command rather than relying on the user prompt handler. So, I'm not sure when the user prompt handler is actually useful.

Speaking to implementors' concerns, you can't interact with a page further when an alert shows up, so aborting the command keeps the state of the session sane. Interaction state (such as a gesture recognizer) is not normally maintained when an alert pops up, so it would be weird to do so for Perform Actions.

Start by reading the WebDriver specification sections covering Perform Actions, user prompt handlers, and the unexpected alert open error. Compare the current command and subsequent-command behavior described in the issue. Done means the specification clearly defines whether Perform Actions fails immediately and when the user prompt handler is invoked.