The save button in the Online UI should indicate if the document opened is already saved or not - exactly how we do in desktop version (some kind of paint over the icon) This bug is also somewhat related to Bug 104156 and code pointers are in comment4 there.
With this commit "https://gerrit.libreoffice.org/#/c/53558/" ; 'save' button changes when there is unsaved changes.
ozturkemre committed a patch related to this issue. It has been pushed to "master": http://cgit.freedesktop.org/libreoffice/online/commit/?id=c879049f1f77484317b68082fefad9b452866f20 tdf#115310 "Save button" changes while there is unsaved changes
Thanks Emre for working on this. This looks as desired now.
ozturkemre committed a patch related to this issue. It has been pushed to "libreoffice-6-0": http://cgit.freedesktop.org/libreoffice/online/commit/?id=91bae92f1c1b1c4cd850de4c64cc6a6f9ea50750&h=libreoffice-6-0 tdf#115310 "Save button" changes while there is unsaved changes