I have a new Doogee y6 piano black and it has a grey round button on the screen that whn i tap on it opens a smaal menu arount it to enable game mode, read mode, float music, float video.
Apparently there is a tutorial available for this float mode thingie, but it is in Chinese... Doogee Y300 Review
Look in Settings -> Device. In the screenshots at the above link, you will see an option called "Float gesture" - I think you want to try to disable it.