fix: scroll focused item to be first visible item after pagedown

[bwajtr]

Description

Pressing PageDown (Fn + Arrow Down on MacBooks) in the Grid should do the following, according to the documentation:

Moves cell focus down by one page of visible rows

The current implementation does that correctly. However, the position of the newly focused item is sometimes at the bottom of the grid, and sometimes, it's the first item at the top of the grid. Besides the obvious inconsistency in behavior, it may also happen to you that you "skip" some items while scrolling the grid using PageDown. Notice that in the following video, you never see items 8 to 12, when pressing PageDown:

Zaznam.obrazovky.2024-01-04.v.18.02.00.mov

This PR changes PageDown behavior so the currently focused item is scrolled to the top of the grid, so it becomes the first visible item. Only then is the focus moved to the last fully visible item in the grid. This way, the user can be sure that none of the items were skipped.

Implementation note: Calling scrollToIndex(currentRowIndex) before counting the visible items makes PageDown work better when the grid has variable height rows.

Fixes vaadin/flow-components#5794

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/contributing/overview
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.
• I have not completed some of the steps above and my pull request can be closed immediately.

[sonarqubecloud]

Quality Gate passed

Kudos, no new issues were introduced!

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

See analysis details on SonarCloud