In a bit of a pickle right now. In Classic, one of our developers made a Visualforce detail button on the Contact object. The VF page referenced is merely a redirect to an external site, but its controller extension passes values to the external site based on the currently viewed Contact record.
However, since Salesforce Lightning just can't have this button, I had to create a new Custom Action, and I referenced the VF page in it. It should work, right? Nope -- when I click the button in Firefox or Chrome, I get a "could not connect" error, and when I open it in IE, I initially get a screen saying "the publisher of this content does not let it open in a frame, try opening in a new window". Only after clicking "Try opening in a new window" does the external site load as expected.
My testing leads me to the following assumptions:
The page CAN load from this button
The page can ONLY load from this button if the browser is instructed to do so in a new window
Therefore, I need a way to force the Visualforce Custom Action to open in a new window, or else this action won't work in Firefox or Chrome, and will only work with an additional step in IE.