const VERIFICATION_PIPELINE_SCALING_MULTIPLIER: usize = 2;
The extra number of blocks is calculated using lookahead_limit * VERIFICATION_PIPELINE_SCALING_MULTIPLIER.
If this capacity is exceeded, the downloader will start failing download blocks with BlockDownloadVerifyError::AboveLookaheadHeightLimit, and the syncer will reset.
Since the syncer queue is limited to the lookahead_limit, the rest of the capacity is reserved for the other queues. There is no reserved capacity for the syncer queue: if the other queues stay full, the syncer will eventually time out and reset.