have game auto resize to smaller resolution if browser window size is resized to be too small for the set resolution to fix this.

Fixed and shipped, and built the way you suggested in your own comment.

The cause: picking a higher resolution here makes the canvas genuinely BIGGER — the UI keeps its authentic 800x600-authored pixel sizes and is re-anchored rather than rescaled, which is the whole point of the HD mode. But when the browser window is smaller than the resolution you picked, the canvas cannot be shown at 1:1 and the scaler has no choice except a continuous downscale of the whole thing. At that point 11px chat text lands at about 5-6px. Nothing was wrong with the chat; the whole canvas was being squeezed.

Your pick is now a CEILING rather than a demand. The game uses the largest of the five resolutions your window can actually display at 1:1, and steps back up on its own the moment you make the window bigger — no setting to change, nothing to remember. 800x600 stays the floor, where letterboxing is the right answer.

The same rule is applied at boot and on every resize, deliberately: the two disagreeing is what made the old "Expand View" setting silently reset itself a frame after boot.

LIVE on the published build. Try it at 1920x1080 in a half-width window — the chat should stay readable and you should simply see less world.