Fix ConditionalBlock crash in manuscript projects

[cderv]

ConditionalBlock in manuscript projects would crash with a fatal error during rendering when using conditional content like .content-visible when-format.

Summary below, more details in

• Custom Blocks Div like Conditional Blocks are throwing fatal error in Manuscript project #13616 (comment)

Root Cause

The manuscript filter built block collections using pandoc.List() and assigned them to divEl.content. This changed the content type from "Blocks" to "List". When ConditionalBlock's render function returned el.content, it returned a List without a .walk() method.

During custom node rendering, has_custom_nodes() attempts to walk the rendered result to check for nested custom nodes. The AST walker's checked_walk() function expects walkable Pandoc elements (Blocks, Inlines, Pandoc) and fails on plain Lua tables like List. This guard rail correctly caught the type mismatch.

Why Manuscript Specifically?

Default projects don't run the manuscript filter, so Div.content remains type "Blocks" throughout. Only manuscript's flattening logic (which builds new block collections) exposed this type mismatch issue.

Fix

Changed manuscript.lua to use pandoc.Blocks({}) instead of pandoc.List() to maintain proper type throughout the filter chain.

Fixes #13616

A note that this bug was introduced in 1.7 and it was not failing in 1.6. So maybe we should consider backport... 🤔

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