Online source editor needs to popup with warning if page is navigated away from

When I'm editing a source file in Bitbucket using the web client at bitbucket.org, if I accidentally hit refresh or previous page, I lose all the changes that I haven't committed yet. The vast majority of websites, when you're editing something, whether that's code, or a text document, spreadsheet, blog post, status update, whatever, when you press refresh/previous page/next page on your web browser, the website popups with an alert box to tell you haven't finished your editing, are you sure you want to navigate away. And then you get an option for yes I'm sure, do navigate away and an option for no, I'm still editing, don't navigate away. This alert gives you a chance to save yourself in the event that you didn't really mean to leave or if you weren't aware you didn't save. How does Bitbucket not have this basic functionality? That is horrible user experience. Nobody should ever lose work because of a misclick or accident.