Root cause per QA on palette-dev (merge d402113): splitLines() joined
.codeline spans with '\n'; under white-space:pre-wrap each newline text
node between display:block spans rendered as its own extra visual row,
while renumber() counted gutter rows only from span heights — numbers
drifted up one row per wrapped line. Join with '' (spans are blocks).
Also: in the <=640px media query, .gutter { font-size: 15px } overrode
the paste-view gutter size while .code switched to 13px, so gutline rows
were ~36.7px vs 22.1px code rows at narrow widths — every number below
line 1 drifted. Scope the paste-view gutter to the code font size.
Verified with headless Chromium DOM geometry (desktop 1400x900 and
mobile 375x812): gutter number tops equal their .codeline tops for
lines wrapping to 2-3 rows, wrap OFF numbering unchanged
('1\n2\n3\n4\n5'), no horizontal scroll.
fen
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