Hey, thanks for the report. A couple of things to help us narrow this down:
For now, you can close the window with Alt+F4. It’s still closable, it just has no visible buttons.
For troubleshooting, please check this setting: open Settings with Ctrl+Shift+,, search for window.controlsStyle. If it’s set to custom or hidden, change it to native and fully restart Cursor. On Windows, the Agents Window uses native window controls, and when this isn’t set to native, the buttons can disappear.
Two quick questions so we can find the cause:
In the normal editor window, not the Agents Window, do the three buttons in the top-right show up normally?
What is window.controlsStyle set to right now?
Let me know what you find. If the buttons don’t come back after switching to native, we’ll dig deeper.