fix: allow to set virtualizer items when not opened

[web-padawan]

Description

Fixes a regression from #7277

In vaadin-combo-box, we have a logic that clears scroller items on close:

web-components/packages/combo-box/src/vaadin-combo-box-mixin.js

Lines 507 to 508 in 2efeeeb

	scroller.setProperties({
	items: opened ? items : [],

In the PR linked above, this behavior was broken by the items observer change.

Added a test to ensure visible items count is reset (which is a side effect of clearing items).
This seems to be the easiest way to test the change without accessing __virtualizer API.

Type of change

• Bugfix

[sonarcloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[vaadin-bot]

This ticket/PR has been released with Vaadin 24.4.0.alpha22 and is also targeting the upcoming stable 24.4.0 version.