The flowchart preview pane accumulated raw Mermaid source when the user
fired several addNode mutations within the 250ms debounce window —
mermaid.run({nodes:[div]}) is async, so the previous render's
<div class="mermaid"> (still carrying the source text) sat in
.flowchart-preview-render when the next render cleared the target. The
first render's eventual element.innerHTML=svg landed on a detached node,
but the visible preview pane had a stack of stale <div class="mermaid">
elements carrying the source.
Fixed by switching renderFlowChartMermaid (src/renderer.js) to
replaceChildren() and adding a per-target WeakSet in-flight tracker so
the new render always starts from a clean slate and the previous render's
eventual innerHTML=svg is harmless on a detached node.
Added a second regression test that fires 7 mutations inside the debounce
with a renderMermaid mock that mimics mermaid.run's async innerHTML=svg
closure.
Amit Haridas
amitwh