This might not be the exact same case but it might be helpful.

Workaround for the 'off-screen' issue is to have a long enough text in the test case for whatever widget you are testing - in my case it was a DropdownMenuItem and I've set the text to a very long string: "Really long string to be clicked successfully"

For the DropDownMenuItem, it seems like the items don't fill the width of the menu.