I have just provided a fix for this issue. It is available as part of the issue tracker
Sticky Language Filter
Just download the attached file, script_manifold.js, rename the existing one in your
folder, and place the updated version there.
Sandcastle already has the framework for supporting this in place, just needed some additions
to work with the HTMLHelp 1.x Viewer.
The changes are marked "CHM: Persistence.....", I think in three places.
NB: Now that the Sandcastle is back let the fun begins....