Keep the scroller position while scrolling lazily and with undefined size

[mshabarov]

Fixes vaadin/vaadin-combo-box-flow#386

[CLAassistant]

All committers have signed the CLA.

[web-padawan]

I assume the fix can be tested in ComboBoxFlow, but it would be good to have a test for it here as well.

[tomivirkki]

The scroll position should ideally be restored when only the size gets changed (not when the items is completely assigned a new value, for example on filter).

As it is, the suggested change causes the following UX regression (on filter the index is restored to a random position):

Compare to the behavior in master:

[mshabarov]

As it is, the suggested change causes the following UX regression (on filter the index is restored to a random position):

Good catch, thanks. Indeed, when we apply a filter, it may happen that the _restoreScrollerPosition method is invoked with the obsolete position and results in such a weird random scrolling. I fixed this by setting oldScrollPosition to 0 so as to avoid execution this scroll pos restore code.

The scroll position should ideally be restored when only the size gets changed (not when the items is completely assigned a new value, for example on filter).

I noticed the methods _getItems and _restoreScrollerPosition are invoked only when the size is changed actually. If you use a defined size mode and just scrolling to the end of list, then it only invoked once at the beginning.

[tomivirkki]

Just tried with the latest revision and it still works the same as before:

[mshabarov]

I made a fix to address this regression with filtering and tested it with basic demo example using large size item array. Unit test has been modified as well, because previously it passed even with this regression. Please review.