CLS verified zero (0.00000) on /blueprints/1 and /overlays/1 via
PerformanceObserver({type: 'layout-shift', buffered: true}) on a
real browser session — previously CLS=0.00859 from a 253 px shift
when cm6 mounted into a display:none slot.
Mechanism:
- editor-entry.js: mount() accepts `rows`. When provided, prepends
an EditorView.theme that pins
.cm-editor { height: calc(rows * 1.84rem + 1.125rem) }
and sets .cm-scroller overflow:auto. cm6 renders at a fixed,
predictable height; long content scrolls internally (same UX the
raw <textarea rows="N"> used to give).
- editor.js: reads textarea.rows attribute and passes it to mount().
- editor.css: new .editor-mount wrapper uses the same calc on
min-height keyed off an inline --editor-rows CSS custom property,
so the slot is pre-reserved BEFORE cm6 mounts. Wrapper and cm6
match exactly (browser-measured 254 / 254 px for rows=8 and
607 / 607 px for rows=20).
- Templates: each editor textarea wrapped in
<div class="editor-mount" style="--editor-rows: N">. Single source
of truth on N (only the rows attribute + the inline custom prop
vary per call site).
Per-row metric 1.84 rem derived empirically: 253 px for rows=8 minus
1.125 rem chrome = 235 px content, ÷ 8 ≈ 29.4 px = 1.84 rem.
Fast suite + e2e suite still green (3 + 2 pass, 0 fail).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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