Performance Guide¶

Tips for keeping Kodemirror editors responsive, especially with large documents.

How Virtualization Works¶

Kodemirror uses LazyColumn for virtualized rendering — only visible lines are composed. This means:

• Documents with millions of lines are feasible
• Memory usage scales with viewport size, not document size
• Scrolling performance depends on line complexity, not document length

Decoration Performance¶

Decorations (syntax highlighting, markers, search matches) are stored in RangeSet — a sorted, immutable tree structure. Performance tips:

• Keep decoration counts reasonable. Hundreds of decorations per viewport are fine. Tens of thousands may cause slowdowns.
• Use RangeSetBuilder to create decorations in sorted order. Out-of-order insertions are rejected.
• Reuse decoration instances. Create Decoration.mark(...) once and add the same instance to multiple ranges.
• Update incrementally. In ViewPlugin.update, only rebuild decorations when update.docChanged or update.viewportChanged.

override fun update(update: ViewUpdate) {
    if (update.docChanged || update.viewportChanged) {
        decorations = buildDecorations(update.session)
    }
}

Parser Performance¶

The syntax parser runs incrementally — it only parses what's needed for the visible viewport plus a buffer zone.

• ensureSyntaxTree parses up to a given position. Don't call this for the entire document unless necessary.
• forceParsing blocks until parsing completes up to a position. Avoid calling this from the main thread for large ranges.
• Language nesting (e.g., JavaScript inside HTML) adds overhead proportional to the number of nested regions.

StateField vs ViewPlugin¶

Rule of thumb: If you need decorations that depend on the viewport, use ViewPlugin. If you need derived data from the document, use StateField.

Minimizing Recomposition¶

• Avoid reading session.state in composables that don't need it — this triggers recomposition on every transaction.
• Use onChange or onSelection for callbacks instead of observing state directly.
• @Immutable on EditorTheme enables Compose to skip recomposition of theme-dependent composables when the theme hasn't changed.

Large Document Checklist¶

For documents over 100K lines:

1. Use basicSetup or a minimal extension set — avoid unnecessary extensions.
2. Ensure syntax highlighting uses an incremental parser (all built-in languages do).
3. Avoid decorations that span the entire document.
4. Use collapseUnchanged in merge views to hide unchanged regions.
5. Test on target platforms — mobile devices have less memory and slower CPUs.