Normal applications with web UIs (hotmail, wikipedia, most web forms of recent vintage) don't have this problem.
Unsaved Comments are lost when I go back to a page where I had typed in but not saved my input.

Seen in 3.12.2 Pro and here (3.12.3 Enterprise)

How to reproduce.
Start adding a comment to this issue. (I.e. type some stuff into the comment field, but don't hit add.)
Click to create a subtask, and do so.
Come back expecting to be able to save the comments you entered earlier by hitting add.
The comments will have disappeared, s

I am NOT interested in a workaround. Most major apps, like bugzilla, don't have this problem.

Perhaps its an expiration meta tag set by the web server?

In firefox, when using other web apps, I can even type some data into a form, CLOSE the webpage it's on, and re-open it using History...Recently Closed Tabs, and find what I've typed in is still there!