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PF knows about the issue, but can't fix it, as it's at Facebook's end
If you insist to share your posts, for whatever reason, there are 2 ways
Play on Facebook, the issue doesn't occur there
Play on Portal, when the posting dialog shrinks, click it, press Tab (once) and press Enter
What you normally see in a popup, will then be shown over the entire tab, forcing you out of the game
But at least you can post 😉