1. When opening the world selection screen in the snapshot, even when i am not hovering above the "play" triangle icon it tells me to "Make sure you backup this world before playing in this snapshot". I have opened the worlds at least once already. (Screenshot provided)
2. In all versions I have tested this, all of the triangle play buttons appear at the same time instead of appearing only when you hover above the saves icon.
3. After I open the inventory in creative mode, I cannot take any items out without using Pick Block. I also cannot get rid of any items from my inventory without using /clear. ALL SCREENSHOTS PROVIDED BELOW!
4. The same issue above happens with chests, ender chests, etc.
Most probably you have touchscren mode active. Disable it.