Repro: run the game on one display and switch displays whilst the game is running. You cannot choose to change the resolution to the second displays resolution if it is a higher resolution, until after Dota has restarted.

Eg, running first @ 1440x900, switching to a 1080p display, the option for 1920x1080 is not available in options (under 16:9) until after restarting.

Expected: all resolutions for the current display are available or the native resolution is detected and always made available in options.

(this has been an issue for a long time even in the old client)