It appears that we’ve faithfully imported a bug from LL which can cause the viewer to exit silently on Windows (not known whether other platforms are affected). The bug seems to be around mouse handling at the end of a drag operation when the drag is released.
We’ve reproduced the bug in LL’s 6.3.5 and reported it and have switched the default downloads in Sourceforge back to 6.3.4.
Once we’ve had a chance to figure out exactly how the crash is happening we’ll reissue 6.3.5.