The previous retry-on-visibility fix didn't help because the issue isn't visibility — it's the slot reassignment in presenter mode. Only the current and next slides get slotted into named slots; the rest are pulled out of the render tree entirely and lose their layout boxes. Mermaid's text-width measurements then come back NaN and it errors with <g transform="translate(undefined, NaN)">. Fix: expose a window.__morkdeckMermaidReady promise from mermaid.eta and await it before sending presentation.start. Lit's initial render defaults to audience mode (every slide in the default slot, layout boxes present), so mermaid measures successfully; only after every block has its SVG do we flip into presenter mode and reassign slots. The SVGs travel with their slides through the reassignment. Verified the presenter tab now lands a 1109×189 mermaid SVG on initial load, no NaN transforms, no refresh needed. Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>