This is with 8.1.9 running under Ubuntu 20.04.3 LTS
When I invoke the style editor on a style defined under the Styles node of the project browser, the editor window has a title bar that allows me to drag the windows around.
However if I invoke the style editor on a style defined in the properties of an object, the editor doesn’t have a title bar, and is fixed in the location where it appeared. And I can’t move it about in order to see what is underneath it.
The editor really should have a title bar in both cases.