The paste gutter was pinned to a fixed 3ch width. With box-sizing: border-box that leaves only ~19px of content after the 10px+10px side padding, so 2+ digit line numbers overflow right into the code text (owner-visible from line 10, worst at 100+). paste-lines.js now sets the gutter width to calc(Nch + 20px), where N is the digit count of the highest line number, via CSSOM (CSP forbids inline style attributes). Numbers were already right-aligned; the column now matches the width of the biggest number. The width is only written when it changes, so the resize-observer/renumber loop keeps a stable fixed point.