1. In a computing device that implements a video encoder, a method comprising: receiving video frames; and encoding the video frames to produce encoded data in a bitstream, wherein the encoding the video frames includes: as part of processing for a slice header of a slice, evaluating a condition, wherein the condition depends on whether value of a variable is greater than 1, the variable indicating a number of total reference pictures; and depending on whether the condition is satisfied, conditionally signaling in the bitstream a flag that indicates whether a reference picture list (“RPL”) is modified according to syntax elements explicitly signaled in the bitstream.