feat: Mermaid diagram inspect/zoom UX in preview

[AmirTlinov]

Problem

Large Mermaid diagrams in live-preview.nvim are hard to inspect when they are rendered inline in Markdown/AsciiDoc preview. Users currently need browser zoom hacks or horizontal scrolling, which is slow and inconsistent.

Goal

Add a first-class inspect/zoom UX for Mermaid diagrams so users can quickly open a larger focused view and read complex diagrams comfortably.

Proposed behavior

• Show a per-diagram magnifier button on Mermaid diagrams in preview pages.
• Clicking opens a fullscreen/lightbox modal with the diagram cloned at a readable scale.
• Provide controls:
  • Zoom in
  • Zoom out
  • Reset zoom
  • Close
• Preserve live-preview behavior across updates/rerenders.

Scope

In scope

• Mermaid diagrams rendered in preview output (Markdown + AsciiDoc where Mermaid is supported).
• UI controls and modal styling injected by preview runtime.
• Idempotent runtime behavior (no duplicated controls/modals on rerender).

Out of scope

• Global page zoom behavior.
• Non-Mermaid content zooming.
• New plugin dependencies.

Acceptance criteria

• Each Mermaid diagram has an inspect (magnifier) trigger.
• Clicking inspect opens a modal/lightbox with enlarged diagram.
• Zoom in/out/reset/close actions work reliably.
• Works for both primary Mermaid rendering path and fallback rendering path.
• No duplicated controls/modals/styles after multiple live updates.
• No Neovim startup regressions.

Test plan

• Manual: Markdown/AsciiDoc files with small and very large Mermaid diagrams.
• Runtime smoke: verify modal creation, controls wiring, and idempotency on repeated rerenders.
• Regression: ensure existing :LivePreview start/close/pick flow is unaffected.

References

• Candidate implementation PR: feat: add mermaid zoom modal controls #358

[github-actions]

Thank you for your issue! We appreciate your feedback and will look into it as soon as possible. Please make sure to include all necessary details (including a minimal config that can reproduce the error) to help us assist you better. Pull requests are also welcome!

[AmirTlinov]

Opened implementation PR: #361\n\nThis supersedes the previously closed #358 and keeps tracking under this issue.

[brianhuster]

The plan seems good to me

[AmirTlinov]

Мне кажется, этот план хорош.

thanks)