To mitigate this potential, phpBB should have an OnBeforeUnload-type handler that will catch when the current message entry or edit box hasn't been committed. And prompt the user with a "Are you sure you want to leave this page? Your current message will be lost."-type message, to alert them to the fact that message content is about to become abandoned.
It won't change the workflow for anyone who is successfully posting, but will give an extra step to anyone who is intentionally or unintentionally about to abandon an in-progress edit or an in-progress reply.
The scenario that benefits the most is that "Post Review" scenario, where in phpBB it "somewhat looks like" your post actually was accepted, and you're back to reading the thread again. But in reality its showing you the new posts created and your still-in-progress message editor is somewhere lower down on the page now. By adding a prompt, even though the user may not be able to see their in-progress edit area, they will be alerted to it when they try to leave the page.
- Yes, it's been rejected before, on the basis that "I closed it because I know what I'm doing, please don't annoy me." The target audience here are people who don't know they were just sent to post review, or didn't intend to close the tab. Avoiding "unintentional data loss" is worth some effort. Make it configurable to opt-out from the UCP for people who find "prompts" are more annoying than "data loss". But default to helping prevent unintentional data loss.
- Yes, if you knew to hit "Save as a draft" before the data loss, you would still have the message content in your draft. The target audience doesn't know they're about to lose their in-progress message, and won't know to save that draft for the same reason they don't know they're about to lose their data.
- Yes, if the board was running an "automatically save drafts" extension, it might have saved the user's in-progress message at some point before it was lost. The user isn't in control of whether the phpBB board is running such an extension, but the user's time and effort is what will be lost when this situation happens.