A question for the group: (apologies that similar questions have almost certainly been asked but I have seen different answers and not seen exactly my scenario). It's to do with existing custom buttons in LEx.

In Classic we have several custom detail page buttons that launch screen Flows with a parameter passed in from the record. They open as a new window and this works well as the user often needs to review info from the record and record text in the Flow. The custom buttons are still present in the LEx UI but the Flows do not launch properly. When clicking the button in LEx, a new browser tab opens with "Looks like there's a problem. This record is no longer available. Ask your administrator for help".

I have created an Action to launch the same Flow and that works fine, but opens in a modal, so we lose the ability to navigate around the record while the Flow is still running.

Is there a way to have the same behaviour (Screen Flow in a new window or a new browser tab) with either the existing custom button or a newly created Action?

Bonus question: one of our buttons executes Javascript which determines which Flow to launch based on an attribute of the record. The existing button isn't available in LEx and we can't create a new action that executes Javascript, right? What would the best practice be? Have Flow determine which Flow to launch, or something else?

Thanks for reading.

Martin