As you click on "Accept cookies" or close an advertisement popup, you don't realize you're actually clicking on the various locations in the Bitwarden widgets, selecting an offered entry. Toth points out that it is easy for the malicious code to move the add-on's hidden widget to keep it strategically placed under where you're clicking. So, unless Bitwarden is preventing their widget from being hidden (so that you know you're clicking on it), there is a risk.
1Password's designers just assume that someone will eventually figure out a new way to hide an add-on's widget. So instead 1Password offers you the option to turn on a confirmation dialog. The confirmation dialog that 1Password triggers is not running within the web rendering engine at all. It can't be hidden by malicious code on the web site. My understanding is that when that dialog pops up, all execution within the web page is suspended until you respond to it. The extension's web elements trigger code which probably calls this:
window.confirm() instructs the browser to display a dialog with an optional message, and to wait until the user either confirms or cancels the dialog.
developer.mozilla.org