Constant ANCHOR_PRESET 

const ANCHOR_PRESET: i8 = 10;

The preset a frame predicting from an anchor alone is coded at. At preset 11 the library keeps some blocks the picture changed since the anchor as they stand in the anchor, a stale copy every client shows (test_slow_page’s bar, 14 frames back, left where it was in a 64-pixel block); preset 10 codes them. A session naming its references opens at it, since the library takes a preset change only toward a faster one, and codes every other frame at PRESET.