Function front_of 

fn front_of(
    messages: &[Message],
    sizes: &[usize],
    measured_from: usize,
    budget: Budget,
) -> usize

Where the window’s front moves to in messages: the first frame of the oldest turn it keeps.

The window ends at the end of a turn, so its front is a turn’s first frame. line is where a count-based cut would land - len less the cap - and the front is the first turn open at or above it, which keeps the window whole turns and can come in slightly under the cap. The byte ceiling lands the same way: its own line is the first frame whose tail weighs no more than the ceiling, and the cut is made at whichever line reaches later, so what a drop leaves is inside both ceilings or a turn short of the later one. (A copy grows toward a ceiling’s slack again between drops, and the append at its edge rides one frame past before the next push checks - the shape the count alone has always had.)

A turn crossing the line rides into the slack. The newest turn holds the newest frames, so a window cannot drop it: when it alone is longer than the cap, the window keeps it whole up to the cap’s slack rather than cutting inside it - and only when that single turn alone would push the window past the slack is the cut made inside it, the one case with no whole turn to land on. crate::conversation_turns answers where the turns open, and the walk stops at the first one at or above the line.

The window is never left empty. A single frame heavier than the whole byte budget keeps the window at that one frame rather than at none: the newest frame cannot be dropped - a page would open on nothing - and the appends that follow shed it as soon as the tail fits again.

sizes is what each frame of history[measured_from..] weighs; nothing older than that was measured, and nothing older than it can be kept (the ceilings that would reach further back are inside the measure’s own stop).