New Windows Appear on Panel Out of Order

[kris84]

I've used LL 3.2 in the past (on this same computer and for about a year) and after a short excursion trying some other distros, I decided to return to LL. I installed the 3.2 iso (64-bit) that I already had and upgraded from there to 3.8, installing all updates available as I went. I've almost got the system set up the way I want, but I notice that the items in the panel don't appear from left to right as I open them. For example, in this session, I've opened (1) Thunderbird, then (2) Thunar, which shows up to the left of TB, (3) Firefox, which shows up to the left of Thunar. You'd think this was just the reverse of what it should be, but when I went to my Documents folder and opened an odt file, it showed up between Firefox and Thunar. The order is Firefox, LibreOffice, Thunar, Thunderbird, but it should be: Thunderbird, Thunar, Firefox, LibreOffice. I've looked in the Panel Preferences, but can't see anything unusual there. Have I missed an important setting somewhere that's causing this?