Migration guide from v2 to v3

[Taifunov]

Hello, would you add a migration guide how to migrate pane from v2 to v3?

[tomkp]

The migration guide has been updated to include documentation for v2.x users.

See MIGRATION.md which now has dedicated sections for:

• Migrating from v0.1.x - for users on the older API without Pane components
• Migrating from v2.x - for users who were already using Pane components with initialSize

Key differences documented for v2.x → v3:

• initialSize → defaultSize
• split="vertical" → direction="horizontal" (terminology swapped)
• onChange → onResize (with updated callback signature)
• Import path changes

Thanks for the suggestion!