Steps to reproduce:
- In any of the Header, Footer, or No results behavior areas of any view, click the Add button
- Select either Global: Text area or Global: Unfiltered text and click Apply (all displays)
For Global: Unfiltered text the text area is to the right of the Available global token replacements fieldset and outside of the window (you can scroll to the right to see it).
For Global: Text area, both Basic HTML and Full HTML display their text areas in the correct position, except that the Available global token replacements fieldset is overlayed on the field. Also, their WYSIWYG buttons are in a line down the right side of the text area and outside the window (you cannot scroll to the right to see them). The Restricted HTML text format behaves the same as the Global: Unfiltered text field.
Both of them display correctly, if they are not displayed in a modal window (right-click and open in new tab or window).
This may not be a Views bug, but I'm having a hard time finding any other place in D8 that would have a similar setup.