>>> My Info: Win7_64, Nightly 47, 32bit, ID 20160218030349 STR: 1. Open new window 2.A) Open many tabs to cause overflow in tabs toolbar 2.B) Open as many tabs as possible w/o overflow in tabs toolbar [I will refer to the <tab> element which is the last tab after Step 2 as <T> ] 3. Middle-click penultimate tab [don't move mouse pointer - wait until the last tab finishes sliding to the left] 4. Move mouse pointer to the right, to the same place where <T> was placed before Step 3. AR: Titlebar shown: After Step 4 tabs stay still. Titlebar hidden: After Step 4 tabs (A) move to the right (B) resize ER: Those results should be the same. Preferably: "After Step 4 tabs should stay still" Notes: 1) This bug is long-standing. I'm adding bug 1219215 in "See also" list, because it looks similar. I was hoping that bug 1219215 will improve many things about "Titlebar shown/hidden" parity 2) On the screencast you can see Menu bar displayed. I did that only to demonstrate that I move mouse pointer over the empty place inside <tabs> element, not outside of it. Screencast (What see bug 1199185. Please don't file it): > https://dl.dropboxusercontent.com/s/j53ex72o1q8htid/screencast%201%20-%20Tabs%20resize%20or%20move%20when%20I%20hover%20mouse%20over%20empty%20space.webm?dl=0