I did not have "open panel automatically" checked. I don't know if it was checked before and I unchecked it but I don't think I changed anything.
This still seems different from what I remember though. I remember it being a lot more seamless, but now the panel quite obviously isn't in the mail tab when I first switch to it, as it takes half a second to appear. If I then switch back to another tab, the panel is there for a brief moment, and then disappears. This causes YouTube to recalculate it's layout 3 times, and when scrolled down on the front page a bit, it takes it about 2 seconds to do so. I remember the tab switches being a lot faster, more like if I had switched between any two ordinary web sites, like say YouTube and Gmail.
...but, regardless, I can keep an eye on my email and RSS feeds now, so thank you for your help.