Title says what the bug is. Paste any text or command into a command block and the "Done" button becomes unavailable. It requires typing or erasing a character to re-enable it.

To reproduce:
1. Find a command. Copy it.
2. Open the command block GUI. Note that the "Done" button should be enabled.
3. Paste the copied text into the command line. The "Done" button should disable.

To Re-enable:
Type or erase a character. The Done button will re-enable.

I will also mention that when the command line is empty, and you type one character, the button is also disabled. typing a second re-enables it, but strangely erasing the second character and leaving only one leaves the button enabled.

It's a minor thing that kinda bothers me.