fix: do not re-render when setting sync property to same value

[vursen]

Description

Reassigning a sync property to the same value should not trigger a Lit re-render.

Fixes #8221

Type of change

• Bugfix

[vursen]

note: Lit supports customizing property change detection with hasChanged. However, I decided that adding it to PolylitMixin is not useful since Polymer doesn't support this feature anyway.

[vursen]

Some of the failures make sense – they reveal that certain properties lack sync: true, for example:

web-components/packages/overlay/src/vaadin-overlay-mixin.js

Lines 17 to 28 in 9e08816

	class OverlayMixin extends OverlayFocusMixin(OverlayStackMixin(superClass)) {
	static get properties() {
	return {
	/**
	* When true, the overlay is visible and attached to body.
	*/
	opened: {
	type: Boolean,
	notify: true,
	observer: '_openedChanged',
	reflectToAttribute: true,
	},

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[tomivirkki]

Is this change related? Couldn't find which package is affected

[vursen]

As far as I noticed, some tests finish before the resize observer processes the changes they triggered (at least setTimeout adds a delay). If the observer does something that triggers a Lit update, Lit throws an exception because the element no longer exists by that moment. This exception only appears in the logs and doesn't cause a failure because the test has already finished. Let me know if you would like me to exclude this change from the scope of this PR.