I did forget when Apple introduced the automatically hidden scrollbars. Why does LibreOffice still doesn't support that? Those scrollbars at the Mac version are very ugly.
Because LibreOffice is not a fully implemented Cocoa application. It relies on its own UI framework for drawing widgets, and windows make use of the only the most basic of NS Objective-C classes without implementing all the niceties of a "true" OSX app.
Valid request, setting to enhancement.