When I play Minecraft Classic, sometimes the menu screen appears when mining/hitting/placing blocks. It starts to get annoying when you are trying to make a giant house. So my question is why does it appear, and how to remove/prevent it.
Bit of a late answer, but it's because the client is somehow losing focus when clicking. This is mostly because of Java changing something in mouse handling, and the older LWJGL not expecting a change.
You can downgrade to java6 to play it without this issue.
This happens to everyone. It happens to me too, and I think its because of keybinds. I think Classic doesnt have a grasp on which key is which, and it is a good thing that Notch udpated MC to prevent this. Do you really wanna play on classic? Why not a beta version if you wanna play old-school?