So take a look at http://www.qualificationcheck.com under both Chrome and Firefox. Alt-tab rapidly back and forth between them focusing on that little green ‘help & feedback’ side tab.
It appears to move position! Whys this?
Its included by a 3rd party Javascript file. I’ve looked into it to figure out how it calculates its position.
First it sets top: 50% to get it roughly 50% of the way down the viewport.
Then it sets
margin-top: [ "-",Math.round(tab.dimensions.height / 2), "px" ].join("")
ie minus half the height of the tab so it shifts back upwards slightly so the ‘middle’ of it is actually 50% of the way down the viewport (rather than the ‘top’).
Using Chrome dev tools and then firebug in Firefox I can see that in Chrome margin-top ends up being -33px while in Firefox it ends up being -87px.
Why the difference?
Its annoying because I want to add my own tab above or below it but I can’t determine where to put my own tab if I can’t rely on the 3rd-party one to be in the same position all the time!
Sorry guys, neither of the other answers helped.
I resolved this by basically copying the 3rd party js and then tweaking it so I could position it and my new tab together as one.
So basically just a work around rather than an answer to the issue.