seems I was wrong, using proper measurement it comes out to 1170px. which is still lower than 1280, but 1170 is for the view port, your browser also takes up room in borders/bars/UI. i doubt that accounts for 110px but still.
(i moved best answer to this one, feel free to unmark)
Actually I think the problem is the borders. If you look at the image, you’ll see a blue border next to the “TrinityCore” button and one on the other side next to the “Open Quick Navigation”. Those are probably enough on smaller resolutions to squish the buttons together.