I'm currently using 2.8.2 but this also happens with 2.8.1 when I hit F12 for render it opens a new window, can't seem to find any options to have it display the render result in the current ui layout instead I get a full 1:1 pop up of my render taking over my screen I rather have the render inside of one my window panels. Has this been changed or just removed?
In the Render menu under the Display mode sub-menu you can select various options.
Full Screen - Images are rendered in a maximized Image editor.
Image Editor - Images are rendered in an Image editor.
New Window Images are rendered in a new window.
Keep User Interface - Images are rendered without changing the user interface.
For more information see top bar documentation