styles are not read from the constructor chain

[tlouisse]

Description

Live Demo

https://stackblitz.com/edit/lit-element-request-update-bug-lspgdq?file=test.js

Steps to Reproduce

1. Create function MyStyleMixin with static get styles defined. For instance:

const MyStyleMixin = (superclass => class MyStyleMixin extends superclass {
  static get styles() { return [
    css`
      div {
        border: 2px solid blue;
      }`
    ];
  }
});

2. Create class MyStyledElement that applies MyStyleMixin and extends LitElement.
   Define a render function that are styled by MyStyleMixin. Define it as custom element my-styled-element.
   For instance :

class MyStyledElement extends MyStyleMixin(LitElement) {
  render() {
    return html`<div>Styles should be applied here</div>`;
  }  
}
customElements.define('my-styled-element', MyStyledElement);

3. Instantiate <my-styled-element></my-styled-element> on a page.

Expected Results

<my-styled-element> has a div with a blue 2px border.

Actual Results

<my-styled-element> has no styling at all.

Browsers Affected

• Chrome
• Firefox
• Edge
• Safari 11
• Safari 10
• IE 11

Versions

• lit-element: v2.0.0-rc.3
• webcomponents: v2.2.3

[web-padawan]

I have also noticed that while working on web-padawan/lit-components#30. Had to tweak the code when updating to 2.0.0-rc.3 and make sure the class extension in the tests does the following:

static get styles() {
  return super.styles;
}

For me it was an acceptable tradeoff, but in general it looks like a regression.

[tlouisse]

@web-padawan : Yeah, I also already did this in my code as a workaround :)
But we have quite a few mixins that share styles and it feels a bit hacky to me if we have to trigger the LitElement like this everytime.

[dfreedm]

The intent is to inherit styles if they have not been set on the subclass:
https://github.com/Polymer/lit-element/blob/master/src/lit-element.ts#L75

[tlouisse]

Thanks for fixing the issue @azakus, pretty smart to check every level via finalize 👍 Was there a particular reason to rename it from _finalizeto finalizeif I may ask?