This happens regularly to me, too. When I edit a note in a separate window, what I've written is often lost when I close the note. It's really frustrating and leading me to consider moving to an alternative note-taking system.
one of the problems is that issue; right hand pane and standlone note window. After the first experience, i took my notes again in the right hand pane. But it's a small pane, you know, i made some arrengements with standalone note window. For example, i made some sentences red color, but at the same time it was still black in right hand pane. i closed standalone note window but everything was same (black color). i opened standalone window and all my arrangements (colors etc.) was gone.
i tried ctrl-z but it didn't work. i can email you but i don't know how i form the thread link.
Yes, we're aware of an issue where, if you edit in a separate note window but leave the main note open, the contents can sometimes get out of sync, which can result in the contents of the separate window being reverted. We'll have a fix for that in the next version (possibly by removing the note from the right-hand pane while the separate window is open). For now, as far as we know, if you click away to another item in the middle pane after opening the separate note window, you shouldn't have any trouble.
Problems with Undo in notes is a separate issue that we'll try to fix soon. The main issue for this thread was about the note editor in the right-hand pane and in the separate window getting out of sync, possibly resulting in lost content when the separate window was closed. That should no longer be an issue in 5.0.31 and above, which no longer displays a selected note in the right-hand pane if the note is open in a separate window.
Upgrade Storage
Otherwise, if you email us at support@zotero.org with the first line of the note and a link to this thread, we may be able to restore it for you.
Also, was this in the right-hand note pane or the standalone note window?
i tried ctrl-z but it didn't work. i can email you but i don't know how i form the thread link.
(This thread seems to have multiple duplicates such as: https://forums.zotero.org/discussion/57670/accidentally-deleted-notes )
ps: don't give up Zotero's team, you will eventually find the origin of that bug!