Frontend & browser·Rendering and the browser pipeline
you changed one element's width and the browser had to recompute the position of half the page.
Reflow
Also calledlayout, relayout
The stage that computes the geometry of every box: size and position, in document order, with each element's result feeding the next. It is the expensive stage, because a change near the top of the tree cascades down through everything after it. Anything that changes geometry — width, padding, font size, inserting a node, changing text — triggers it, and on a large tree it costs milliseconds you cannot spare inside a frame.