Unify preview scaling model across vertex, edge, and label previews

[kmcginnes]

Task

The cytoscape-matching previews scale their geometry with two separate, differently-named model→pixel constants, plus a per-call-site CSS zoom fit control. This works but has no single, obvious rule.

Current state:

• Vertex family (VertexSymbol): PREVIEW_SCALE = VIEWBOX(96) / CANVAS_NODE_SIZE(24) = 4. Canvas-unit lengths (border width) and the label (scale={PREVIEW_SCALE}) multiply by it. On-screen size set by size-* classes.
• Edge family (EdgeLinePreview / LabelPreview): EDGE_PREVIEW_SCALE = 2 (renamed from DEFAULT_EDGE_PREVIEW_ZOOM). Different factor (2 vs 4) than the vertex family.
• Composite fit: both EdgePreview and VertexPreview are additionally scaled at the call site with a non-standard CSS zoom-* class (vertex zoom-50; edges zoom-90/zoom-75/zoom-65) to fit their surface. zoom is chosen deliberately over transform: scale() because it affects layout (the surrounding grid/flexbox reflows around the scaled preview).

The result: a 2-unit edge line and a 2-unit vertex border are NOT drawn at the same on-screen scale across previews, and there are two names for one concept.

Goal

A single, documented preview-scaling model:

• One shared CANVAS_PREVIEW_SCALE (model→px) consumed by VertexSymbol, EdgeLinePreview, and LabelPreview, so every preview shares one coordinate system and a given cytoscape unit looks identical everywhere.
• Set on-screen display size only via size-* / width-height (as the vertex family already does), reducing or eliminating reliance on the non-standard zoom property where a size class suffices.
• Decide whether the composite fit-to-surface control stays as zoom (documented as intentionally layout-affecting) or is expressed another way.

Considerations / risks

• Reconciling the 4× (vertex) vs 2× (edge) factors requires re-tuning and visually re-verifying every preview surface (node/edge style dialogs, style import cards, edge details sidebar, schema sidebar, graph viewer overlays). Easy to get subtly wrong — verify in the running app.
• zoom on EdgePreview currently scales the vertex placeholders too; any replacement must preserve "fit the whole composite," not just the inner line.

Related

Follow-up to the edge line/arrow dedup work (#1985, #1986). The conservative step (consistent constant naming + corrected docs) ships with #1986; this issue is the full unification.

Important

Internal only — this issue is maintained by the core team and is not accepting external contributions.