This looks very much to me like a problem many people have had with fixed backgrounds (of which parallax is one).
The webkit installation on your machine seems to get broken by certain plugins added to Chrome. The result is that anything you publish from that machine will do as you describe with fixed backgrounds.
I must stress that this problem occurs due to the installation on the machine from which the page is published but is manifested for all viewers… The same project file will work when published from another, unaffected machine.
Firstly, test if this is the case by publishing a simple page without parallax but containing a fixed background image using a stack such as Joes Jack or my Sections stack. Make sure that the fixed image is far enough down the page so that all of it is not within the viewport on page load.
Look at the resulting page in Chrome - does it suffer the same problem? If so, you will notice that any part of the image not in the viewport at load will not be displayed until you re-size the browser.
This happens because Chrome tries to save power by ignoring fixed image processing for areas that are not in the viewport. Clearly the problem above means that this is not working correctly.
If you verify that this is indeed the problem there are two possible solutions:
Uninstall Chrome and all its plugins completely from your machine. Restart and re-install Chrome.
This has worked for about 50% of people.
Re-install OSX from the Apple combo update. (You will not loose any data - although obviously back up first anyway)
This has worked for 100% of people where option 1 did not resolve the problem
Here is the link to the 10.11.3 combo update - you may of course need a different one depending on your current OSX version. https://support.apple.com/kb/DL1859?locale=en_US
I know this ( apologies for the over simplification) and I know that it doesn’t make sense but we have had well over 30 users who have experienced this problem over the past 6 months. In each case either a Chrome re-install or a combo update has fixed it.
I myself had an affected machine. Joe and I did comparative tests exporting the exact same file from different machines, one suffered the fixed background problems, one didn’t. We never really got to the bottom of what caused it but found a solution for users which has worked in all cases eventually. We even swapped local exported files and diff’ed them revealing no difference. One worked when uploaded via ftp, one didn’t - regardless of the server used.
I can understand how an OSX reinstall can fix potential problems with exporting but in fact almost all users have instantly fixed the problem with just a Chrome wipe and re-install. Again, I know this is anecdotal but the number of people for whom it has worked for point to some validity.
@Chrisfrench The easy way to tell if this is the problem would be to send a simple project to either myself or Joes support emails and we can publish it. If this works then we know the problem is localised to your machine. It may well be that there is some other issue but this exact thing has happened so many times that it seemed like the most probable cause of your problems.
Just jumping in here to say that I can replicate the issue you’re seeing in Chrome 48.0.2564.116 as well as Firefox 45.0. Whilst the background does load full screen in Safari 9.1, I am not seeing any kind of parallax effect on the page (or any of the other pages)—nor am I seeing it once I have resized the window in any of the other browsers.
The top value is being changed on .parallax-mirror and the translate3D value is being changed on .parallax-slider when I scroll but that isn’t doing anything visible on the page. What I’m seeing at the moment in all browsers just looks like a background image set to cover on the body (though it isn’t).
This is why THIS forum is the greatest you can ask for help and everyone joins in. If I can return the favour in anyway, let me know. I’ll be chipping in when I can.
Have to actually agree with @tav, I have seen reinstalling Chrome fix the chrome render issue many times, just this time it was more than just a Chrome render issue.