1. A computing device comprising a processor and memory, wherein the computing device implements a video encoder adapted to perform a method comprising: selectively enabling temporal motion vector (“MV”) prediction for deriving MV information for blocks of a current B-frame in a sequence of video frames; encoding the blocks of the current B-frame, including, for a given block of the blocks of the current B-frame, encoding the given block of the current B-frame using motion compensation with derived MV information relative to a reference frame to determine a motion-compensated prediction for the given block, wherein: if temporal MV prediction is used for the given block of the current B-frame, the derived MV information for the given block of the current B-frame is derived by performing temporal MV prediction based upon MV information of a block of another frame in the sequence of video frames; and if spatial MV prediction is used for the given block of the current B-frame, (1) the derived MV information for the given block of the current B-frame is derived by performing spatial MV prediction based upon MV information for one or more surrounding blocks of the current B-frame, and (2) the reference frame for the given block of the current B-frame is selected from among reference frames used for the one or more surrounding blocks of the current B-frame; and wherein at least one of the blocks of the current B-frame is encoded using motion compensation with derived MV information derived using spatial MV prediction; and outputting encoded data for the blocks of the current B-frame, including signaling information in the encoded data that indicates whether temporal MV prediction is enabled for the deriving the MV information for the blocks of the current B-frame.