I remember when bing used to do this on their mobile site, it would load then I would go to tap on the search box to focus my cursor and just as I was about to do that it would pop in an ad for their mobile app. Then I would have to close out of the play store and get back to actually searching.

Now it seems that protonmail is doing something similar in that I navigate to their mobile site, wait for it to load. Once it's loaded I go to tap the menu icon and just before I do the pop in for their app appears and again off to the play store I go.

I'm not sure though is this considered a dark pattern? But, whatever it is, does this actually end up benefiting them? Do more people end up installing their apps? And if more people do install their app, what is the benefit for them? Is it to save bandwidth or are they able to track their users more closely? If it's the later, why would a company based on privacy and security want or need more information about their users?