The warning that you're navigating away while playing would be more convenient as an alert rather than an overlay. There's no way, save refreshing, to get out of the overlay.
At first I thought you were crazy, but then I realized:
(Note: This is just what I think is happening to Veritas... it might not be this.)
While playing glitch, leave the page. (Preferably go to a slow site from one of your bookmarks.)
Then, quickly hit Escape (or whatever) to stop the new page from loading. You'll still be in the game.
Your character can move around, but there will be a dimmed overlay that says, "WAIT! You were just about to win the game!" At that point, all the game elements can no longer be interacted with, even though the animations may continue. The only way to get out of it, as noted, is to refresh.
However, Veritas, the problem is that that's not a "warning." That's not like on other (mostly Flash) sites where they pop up a confirmation to ask you if you're SURE you want to leave, allowing you to "Cancel."
No, this is just more Glitch-style humor from the dev team. Their assumption is that you're leaving, for sure, since you just told your browser to go somewhere else. Thus, the game shows you the overlay solely as a joke, not expecting you to NOT leave.
I believe this is a browser message. I've seen it in Gmail when I delete a bunch of mails and then immediately switch to another tab while Gmail is still working on the delete. Apparently Gmail needs to keep the page in focus to finish the delete. Not sure if this was Firefox, Chrome or both. I can see this happening in Glitch if you tried to switch tabs while, say, a tree-watering action was still in process. But the message I've seen has buttons to choose to either continue or cancel the change to another page, so it doesn't square with Veritas's not having any way to get out of the dialogue except with a refresh!?!
Yeah, I wouldn't want a confirmation message. There's no penalty for reloading - it just takes a bit of time. A confirmation message that prevents you doing what you (most likely) want to do would be annoying and unnecessary, I think.