My last change in the chapter editor was missing from my export. How do I make sure it is saved?

From version 1.9.24, the chapter editor saves your last change when you leave it, whichever way you leave: the sidebar, the Library, or another chapter. The pill at the top of the editor shows "Saving…", then "Saved"; "Unsaved changes" means a save is still waiting. If a save fails, it shows "Not saved" in red and tells you why, so copy your text somewhere safe before you leave. In an older version, wait until the pill says "Saved" before leaving the editor.