When Sidebar is visible, and the current paragraph's direction is RTL, the tab icon in the top ruler cannot be clicked on with mouse.
I can confirm this bug. Verified on:
LO 22.214.171.124 on Windows 7 (64-bit)
LO 126.96.36.199 on Ubuntu 13.04 (64-bit)
LO 188.8.131.52.alpha0+ Build ID: 8b96cfd6caedbad7b3b79e57421a834f18c5c511
TinderBox: Win-x86@6-debug, Branch:master, Time: 2013-07-27_22:47:00
Changed version to oldest known
If you click once on the tab icon, and then hover the mouse pointer on it, you'll see "Hide comments" tooltip. So I think it's related to Bug 69280 which was fixed recently. Especially note Bug 69280 comment 1. Also the trick from Bug 69280 comment 2 (zooming out) works also in your case.
So would be great if you could test that with latest master builds.