Comment 8 for bug 902054

Based on your screenshot this is the same issue with Ubuntu's custom scrollbar-overlays as reported in Bug #884368.
(Some of Inkscape's GUI elements - which do not use fixed (hard-coded) minimal (px) sizes - seem to get initialized too small if those custom scrollbars are used instead of the default Gtk+ scrollbar: affects also the XML editor, the list of font styles in the 'Text and Fonts…' and the 'Glyphs' dialog, etc.)

Proposing to link as duplicate to bug #884368.

(Whether Inkscape adds support to save the status (and size) of docked dialogs across sessions, and/or parses the list of current layers when opening the layers dialog for the first time in the current session to determine a reasonable minimal height based on the initial number of layers visible in the list - these are different issues already tracked in other reports: bug #171579, bug #597573).