[GNC] Date Completion - sliding 12 month window

[Parke]

Hello,

I have used GnuCash since 2003.

Today I discovered the Edit -> Preferences -> Date/Time -> Date
Completion option.  This is an option I have wanted almost every year
in January when I am entering data from the preceding December.

Unfortunately, I am unable to activate this option.

When I click on the "In a sliding 12-month window ..." option, the
corresponding radio button turns solid grey for slightly more than one
second.  Then the radio button reverts to a solid white color.  (The
"In the current calendar year" option remains selected [i.e. with a
small black circle] the entire time.)

I also tried selecting the "sliding" option in a new set of books.  I
was still unable to activate the "sliding" option.  However, with a
new, empty set of books, the solid grey circle appeared only for a
tiny fraction of a second.

Is there something I am missing?  Or is the sliding date window option
non-functional?

I am running on Ubuntu 18.04, running GnuCash 2.6.19.  I am using the
Openbox window manager.  (In other words, I am using neither Unity nor
Gnome.)  No messages are written to stdout / stderr.

Thank you,

Parke