Window resize after Realm upload shows cancel button

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

1. Open Minecraft in a window
2. Go to Realms, Reset World, Upload World
3. Upload a new world, wait for completion so that 'Done' button is shown
4. Resize window, the done button is replaced with Cancel