When I start a new question, the same happens. I've tried this again, and it seems like the first test was when the answer was already saved by the website, and then it worked. Now I discarded the old question, started with a clean question, and then it happens as well.
When I use the back-button in the browser, it works like expected.
I just tested the same situation with Safari. I should tell you that I have a Macbook with the multitouch touchpad. I do not use the back-button in the browser, but I use a double finger swipe to go back. And that is the cause of the problem. When I use the back-button, the page reacts like expected. The popup appears before leaving the page, and I choose to leave or stay. When I use the swipe, I go back to the previous page, and then the popup appears. If I choose to leave, no problem. If I choose to stay, then the page turns white. The same happens when asking a new question. Writing a comment doesn't cause any issues.