PiT 25.1: Dev bundle regeneration causes Lit version conflict when addon imports @polymer packages

[manolo]

Description

When a Vaadin 25 project includes an addon that imports @polymer/* packages (e.g., @polymer/paper-button), the dev bundle regeneration produces a TypeError: this._$changeProperty is not a function error in the browser console.

The error comes from ConnectionIndicator in @vaadin/common-frontend, which uses Lit 4 decorators. At runtime, the ConnectionIndicator class resolves against Lit 3's ReactiveElement (which lacks _$changeProperty), because the generated package.json forces old Lit versions.

Root Cause

When BundleValidationUtil detects packages not in the pre-built bundle (like @polymer/paper-button), it regenerates the dev bundle. The generated package.json includes:

• lit-element: ^3.2.2 and lit-html: 2.4.0 as direct dependencies
• overrides that pin lit-element and lit-html to those old versions via $lit-element / $lit-html

These conflict with lit: 3.3.2 (from the Vaadin distribution), which internally requires lit-element: ^4.2.0. The result is two versions of ReactiveElement in the bundle: Lit 3's (without _$changeProperty) and Lit 4's (with it). ConnectionIndicator ends up extending the wrong one.

Steps to Reproduce

1. Clone: https://github.com/manolo/vaadin-lit-bug
2. Run mvn spring-boot:run
3. Open http://localhost:8080 in a browser
4. Open browser console

The project is a minimal Vaadin 25.0.7 + Spring Boot 4.0.3 app with a single addon (grid-pagination 4.0.0) that imports @polymer/paper-button.

Expected Behavior

The dev bundle should be generated without Lit version conflicts. The package.json overrides should not force lit-element 3.x when the Vaadin distribution ships lit 3.3.x (which requires lit-element 4.x).

Actual Behavior

Browser console shows:

TypeError: this._$changeProperty is not a function

The generated node_modules contain both lit-element 3.3.3 and 4.2.0, and the bundle includes two ReactiveElement implementations.

Versions

• Vaadin 25.0.7
• Spring Boot 4.0.3
• Java 21
• Tested on macOS (Darwin 24.6.0)