Function estimate_height 

pub fn estimate_height(
    msg: &Message,
    params: &LayoutParams,
    measure: &dyn TextMeasure,
) -> u32

Rough height for a row that has never been laid out.

Feeds the estimated heights in crate::index. Deliberately crude — it only has to keep the scrollbar plausible until the row is actually measured, and being fast matters more than being close, since it runs for every row in the buffer on a resize.

Crude, but not low. The distinction matters because while the view is following the bottom its scroll offset is total - viewport, and the total is this estimate for every row that has not been laid out yet — which always includes the row that was just appended. An under-estimate therefore puts “the bottom” above the real bottom and clips the newest message off the bottom edge. Over-estimating costs a slightly wrong scrollbar thumb until the row is measured, which is the tradeoff crate::index already documents.

Three things it has to get right for that, all of which it once got wrong: a body’s hard newlines (dividing the byte length by the column count answers “how far would this wrap”, not “how many lines does it have”, so a five-line message estimated as one); the width the body actually wraps against, which in indent mode is the content width less the gutter; and the vertical padding a code or image block adds. The avatar floor is the same rule layout_message applies at the end.