Darucas: I'm pretty sure this issue (and global performance and lighting bugs) appears because the game is outdated if you let Galaxy update it or simply install from Galaxy.
I've tried installing with the backup installer and prevented Galaxy from updating/verifying the game files and the game now works fine, and actually looks better (better atmosphere, daytime transitions, some enhanced textures...).
Checking the latest patch changelog, I've noticed all improvements and control changes just don't apply if installed from Galaxy. For example, the left/right arrows should turn the camera instead of strafing, right click should toggle 'look mode' on/off, some UI buttons are better laid out etc. That's the other reason why I assume the Galaxy build is outdated.
So make a copy of your saves (in [your profile]\AppData\LocalLow\Cyan Worlds\realMyst\ ) and try reinstalling with the offline installer, without using Galaxy, neither for installation nor updating (go in the game Galaxy settings to prevent it from updating before uninstalling Real Myst, it's easier to avoid updates this way).
If it helps, I'll report this info to support!
This absolutely, 100% works. Thank you! Noisy sky was gone and the control scheme was updated (no more holding down the right mouse button to pan!). When I relaunched Galaxy after using the manual installer, it did attempt to update, but I was able to abort it. Galaxy now says I have an "unknown version" of the game, so it sounds like something isn't syncing between the version hosted on GOG's offline download servers vs. the Galaxy servers.
