While you are replying to a thread or creating a thread, you will see the "Auto-Saved" icon flash in the lower right corner of the text box. This means your text has been saved. If you leave the page or even close the browser your work is still saved. When you return to that thread you will see the "Restore Auto-Saved Content" in the lower left corner of the text box. Simply click on that button and the last saved version of your text will be returned to the text box.