Feature Description

Add partialCached for the static portion of the sidebar tree to reduce repeated sidebar computation on large docs sites, while keeping per-page sidebar state dynamic.

Problem/Solution

On large sites, sidebar rendering is repeatedly computed across many pages even when the tree is mostly identical.

Here are the metrics from hugo --templateMetrics for my custom theme:

• _partials/sidebar.html: high cumulative template cost across many invocations (e.g. 43.6s cumulative, 454 calls)
• Build wall-clock time is still ~4.2s (Total in 4175 ms), because Hugo renders in parallel

So this is not a contradiction:

• Total = real elapsed build time
• cumulative = summed template execution time across calls

Proposed solution:

1. Split sidebar rendering:

• Cached static tree (section/page hierarchy, sidebar menu structure)
• Uncached page-specific state (active/open, page TOC, .Store.Set effects)

2. Cache only static tree using partialCached with shared keys (not per-page URL), e.g.:

• language
• nav root / section
• relevant sidebar config flags

Expected result:

• Reduced repeated sidebar work
• Non-zero cache hits on sidebar static partial
• No behavior regressions

Alternatives Considered

1. Cache the entire sidebar partial
   Not ideal, because it includes per-page state (.RelPermalink, .IsAncestor, .Fragments.Headings) and side effects.

2. Keep current implementation
   Functionally correct, but does repeated tree traversal at scale.

3. External pre-generated nav data
   Possible but adds complexity and maintenance overhead.

Additional Context

• This request targets Hugo build-time optimization, not frontend runtime speed.
• The sidebar is a strong repeated hotspot in template metrics for large docs structures.
• Since Hugo renders concurrently, cumulative template time should be interpreted as hotspot evidence, not wall-clock build time.