HtmlDialogs.
On SU2017 it seems the DPI scaling within the dialog (pxel ratio for CSS pixels) is rounded to integers, while the sizing of the dialog itself doesn’t. If your system uses a scaling factor of 1,25 the dialogs in SU2017 gets 1,25 times bigger in each dimension, but the content doesn’t scale. If your screen scaling factor is 2 however both the dialog and content appears to scale accordingly (based on TDahl’s screenshot).
In SU2018 scaling behaves as expected (the same float scaling factor being applied to both dialog and content).