Mountain has been rebuilt on a newer version of its engine, so it keeps working on new computers. Your mountain comes with you, and so do your saves on Steam Cloud.
What's new
+ Long-running mountains no longer get stuck in time
+ Much lighter on your computer, especially in the background
+ Full screen fills whichever monitor Mountain is on (menu, 3 key or Alt+Enter)
+ A FLASHES switch in the menu, for anyone sensitive to flashing light
+ Saves when you quit
+ The clouds look the way they were drawn
+ The original artwork is back on the loading screen
+ The camera no longer drifts off into the distance
+ Corrected Chinese and Japanese text
If your computer is too old for this version, the previous one is still there: in your Library, right-click Mountain > Properties > Betas > legacy-2.0.1.