I noticed this while uploading information about another issue I'm experiencing, but if you upload a world when Minecraft is not maximised, and then maximise the screen after the upload is complete, the Done button is replaced with a Cancel button.
Steps to reproduce
- Open Minecraft in a window
- Go to Realms, Reset World, Upload World
- Upload a new world, wait for completion so that 'Done' button is shown
- Resize window, the done button is replaced with Cancel