Unified diagram reading

Context

Before this change, Mermaid had a pan/zoom dialog but loads its engine at page startup and renders a second SVG for the dialog. Local PlantUML loads both engines at startup and offers only horizontal scrolling. Both preserve authored source.

Scope

Share the existing dialog and styles, source viewing/copy, and standalone SVG download. Defer large engines and rendering until a diagram is within 320px of the viewport. Do not change pinned libraries, server-mode PlantUML, other diagram families, the CLI, or ordinary consumers’ Hugo-only build requirements.

Behavior and boundaries

One lightweight scheduler serializes both engines on the main thread. Each engine loads once. Hidden or zero-width containers wait until visible. Theme changes invalidate old results; the open viewer retains its reading position. The viewer reuses a rendered snapshot, supports touch pan/pinch, wheel and keyboard controls, fit, Escape, focus restoration and reduced motion. Source is available even after an error. Copy and export report failure, and downloads report initiation only.

PlantUML stays in Blob images; its SVG is never inserted into the live DOM. SVG exports embed their styling, remove active content and reject external resource dependencies. No includes, themes or optional libraries are downloaded. Browser printing shows a rendered diagram or readable source synchronously; it cannot wait for an asynchronous engine in beforeprint. Dedicated Print, Markdown and RSS retain their existing source fallback.

Compatibility

No configuration or build step is added. Local PlantUML remains opt-in; explicit endpoints retain the legacy renderer. Without IntersectionObserver, bounded scroll/resize checks perform the same visibility test. Source survives disabled JavaScript and blocked assets.

Implementation and acceptance

Extract shared scheduling and viewing from assets/js/mermaid.js; retain engine adapters and the existing dialog partial. Update focused output checkers, runtime tests, real EN/ZH documentation and browser tests. Verify multiple and wide diagrams, hidden tabs/disclosures, rapid themes, errors, blocked resources, source fidelity, standalone exports, touch, keyboard, accessibility and printing. Compare requests before/after.

Decision log

• 2026-10-25: Initial bounded design. Main-thread serialization and Blob isolation are retained. Immediate browser printing uses source for unrendered figures. Local implementation and focused browser acceptance are complete; broader site validation is recorded with the delivery evidence. This behavior ships in OINK 1.4.0.