I noticed (with some amusement) this Github thread dating back to 2019, with additions right up to the present, and prayers for a horizontal rule to be added to OnlyOffice (Docs).
A ‘horizontal rule’ is indeed a very useful feature in documents. I am always looking for it myself in OO and wondering where it could be hidden.
Now that I am wiser (there isn’t one), why is it apparently missing from OnlyOffice Docs?
Hey @mk7z, 
Thanks for bringing the horizontal rule saga back to light!
Unfortunately, I can confirm that this feature won’t make it into v9.2 either. It’s still on the wishlist, but we don’t have an ETA yet (sorry!).
We’ll keep pushing it internally, and as soon as there’s any real progress or timeline, I’ll shout it from the rooftops right here.
Appreciate your patience!
Hi @mk7z ,
In the meantime, have you considered using a Style?
Take a ordinary paragraph (Normal) with no content, and give it a border.
I styled this one as 1.5pt thick, and coloured it #808080, rather than pure black, but that is subject to preference.
And then created a style out of it;
The “Next paragraph style” setting to Normal ensures the following paragraph it also not adorned with the top border.
So, an empty paragraph, with style “HR” creates a Horizontal Rule.
Of course this new style is only local to the current document, but I suspect you could add it to the master document template, which will last until the next upgrade of the software. Take care when editing the master template, take a backup before editing, and all that.
I found this works well for me - nice work around. I’m new to the OnlyOffice ecosystem (my brain keeps wanting to refer to OpenOffice…), and love where it’s heading. There’s still catchup - but I’ll help anyway I can
