[NTG-context] Spurious vertical space above figure

Dear list, Motivated by this question on TeX.SX https://tex.stackexchange.com/questions/370292 I found out that in a full-page figure there is a spurious vertical space of .5\baselineskip above the figure.  I was not able to find out where it comes from.  Is this by design?  MWE is below. Cheers, Henri --- \showframe \starttext \input knuth \startplacefigure   [location={leftpage,none}]   %% remove spurious vertical space   %% don't ask me where it comes from   %\blank[-.5\baselineskip]   \externalfigure     [dummy]     [height=\textheight,width=\textwidth] \stopplacefigure \stoptext