Over on https://github.com/w3c/webdriver/pull/1791 we are going to add a new userPromptHandler capability to the WebDriver session to allow certain prompt handling like beforeunload for WebDriver BiDi. If WebDriver BiDi is not present or this capability is not set, the session needs to fallback to the unhandledPromptBehavior capability.

Actually the userPromptHandler capability was a mistake and we should still use the unhandledPromptBehavior capability. I fixed that in https://github.com/w3c/webdriver/pull/1808.

I'm also going to remove specific Marionette unit tests for the unhandledPromptBehavior capability that are already present in wdspec tests, but only leave basic checks for the capability.

Backed out for causing wdspec failures

• Backout link
• Push with failures
• Failure Log
• Failure line: TEST-UNEXPECTED-FAIL | /webdriver/tests/classic/perform_actions/user_prompts.py | test_dismissed_beforeunload - webdriver.error.UnexpectedAlertOpenException: unexpected alert open (500): Unexpected beforeunload dialog detected. Performed handler "dismiss"

Looks like there is a race condition between closing the opened beforeunload prompt and the alert checks for the next to execute command:
https://treeherder.mozilla.org/logviewer?job_id=459162144&repo=autoland&lineNumber=174837-174843

1716378120924	webdriver::server	DEBUG	-> GET /session/890e4672-cd75-4fca-8f7f-4220cc0c63b8/url
1716378121076	RemoteAgent	TRACE	Received observer notification common-dialog-loaded
1716378121077	Marionette	TRACE	Implicitly accepted "beforeunload" prompt
1716378121117	Marionette	DEBUG	0 -> [0,104,"WebDriver:GetCurrentURL",{}]
1716378121126	RemoteAgent	TRACE	Received event DOMModalDialogClosed
1716378121134	Marionette	DEBUG	0 <- [1,104,{"error":"unexpected alert open","message":"Unexpected beforeunload dialog detected. Performed handler \"dismiss\"","stacktrace":"RemoteError@chrome://remote/content/shared/RemoteError.sys.mjs:8:8\nWebDriverError@chrome://remote/content/shared/webdriver/Er ... iver/Errors.sys.mjs:800:5\nGeckoDriver.prototype._handleUserPrompts@chrome://remote/content/marionette/driver.sys.mjs:2843:11\n","data":{"text":"This page is asking you to confirm that you want to leave — information you’ve entered may not be saved."}},null]
1716378121139	webdriver::server	DEBUG	<- 500 Internal Server Error {"value":{"error":"unexpected alert open","message":"Unexpected beforeunload dialog detected. Pe ... Errors.sys.mjs:800:5\nGeckoDriver.prototype._handleUserPrompts@chrome://remote/content/marionette/driver.sys.mjs:2843:11\n"}}

Maybe we have to special-case this prompt type and wait for the beforeunload prompt got handled before continuing. I'll try to see if I can reproduce by maybe delaying the automatic accepting a bit.

Ok, so for now I would propose that we actually wait in _handleUserPrompt() until the dialog was closed and then return. This might change again when I implement support for WebDriver BiDi but it can be done in a follow-up bug. I pushed a try build to check that this works:

https://treeherder.mozilla.org/jobs?repo=try&revision=bf928be4d1358954b9ba0ddad4fb022695ee50c9

https://hg.mozilla.org/mozilla-central/rev/c4f13e045236