^ For anyone else with this issue - we discussed this on Discord.
Basically, it's not a bug. It comes down to that the panel only allows integer zoom.
The panel can nearly fill the entire screen in low-res, because it can zoom up to 3x while still fitting on screen. The full-width low-res panel is 416px, which can zoom 3x to 1248px (still fits within a 1366px-wide screen).
However, in high-res, it can't zoom at all from 1x. The full-width high-res panel is 832px (exactly double the low-res one, as expected), and even a 2x zoom is too wide - it'd be 1664px.
Aside from "deal with it", the only other workaround is to use the compact panel. This one is 640px wide in high-res, and thus can zoom to 2x (which would be 1280px wide).