This behaviour existed in v1.3 and still is there with v1.5. However it's even more annoying when using the "open in background" feature:
Tabs seem to take focus after the contained page is loaded. So if you gou through the content in the feed tab (Feed Details) with your scrolling wheel and click on a link to open a new tab in background and then want to go on in the feed by using the scroll wheel this will work - for some time. Suddenly, when the page loaded in background is done loading, everything seems to stop. You're now scrolling the "invisible" page in the background.
You can go back to the "Feed Details" tab by pressing "CTRL-TAB" or by clicking on the tab "header", but that's still annoying.
Would be nice to have this fixed.