The bug

If you fill a command block with ■ then the launcher crashes. The game itself (Java) remains open. It's easiest to notice the launcher crashing if you have the launcher log open, but if the launcher log is closed, you can still observe the launcher crashing by using Task Manager on windows or Activity Monitor on macOS.

To reproduce

1. Enable the launcher log in the launcher settings.
2. Launch Minecraft. (I've tested this in 1.16.5 and 21w11a.)

3. /give @p minecraft:command_block

4. Paste this symbol into the command block several times: ■
5. Use ctrl + a on windows or cmd + a on macOS to select all of the text in the command block.
6. Use ctrl + c on windows or cmd + c on macOS to copy all of the text.
7. Use ctrl + v on windows or cmd + v on macOS to paste the text again. Paste several copies of the text.
8. Repeat steps 5 through 7. This will exponentially fill up the command block. Stop repeating steps 5 through 7 when the command block is full.
9. Click "Done".
10. The launcher crashes.