Using Moodle 2.7, and either the default Clean theme or the More theme.
There are not many pop-up windows in Moodle, but using for instance the Question Preview pop-up window I notice a strange behaviour upon resizing that window (on my desktop computer). Up to 987 pixels wide, the contents of the window are displayed as expected (1). When resizing the window's width to 988 pixels or more, an unwanted blank top-margin is displayed (2). Approx. 75 px high.
This problem does not occur with another 2.7 theme such as Essential.