fix: ensure focused month is visible when receiving focus

[vursen]

Description

This PR fixes the bug where a month receiving focus by the keyboard or programmatically sometimes wasn't scrolled into the visible area of the calendar.

An investigation established that the bug is caused by the buffer offset that can be set with the --vaadin-infinite-scroller-buffer-offset CSS variable. This offset isn't taken into account by the revealDate method which leads to computing an erroneously greater number of visible months when an offset is set and, as a result, scrolling doesn't happen. Worth noting that the buffer offset is a non-zero value in both Lumo and Material themes so both themes are affected.

💡tip: When reviewing this PR, enable the "Hide whitespace" option to reduce the diff.

Fixes #1892

Type of change

• Bugfix

Checklist

• I have read the contribution guide: https://vaadin.com/docs-beta/latest/guide/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.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
4 Code Smells

No Coverage information
0.0% Duplication

[vaadin-bot]

Hi @vursen , this commit cannot be picked to 22.0 by this bot, can you take a look and pick it manually?
Error Message: Error: Command failed: git cherry-pick 9a0c2fd
error: could not apply 9a0c2fd... fix: ensure focused month is visible when receiving focus (#3219)
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add ' or 'git rm '
hint: and commit the result with 'git commit'