fix auto-scrolling and highlighting current concept in hierarchy

[osma]

Reasons for creating this PR

The custom scrollbar implementation was removed in PR #1360. But the PR also removed some functionality from the hierarchy tree: highlighting the current concept and automatically scrolling so that the current concept (or at least one of the places where it appears) is visible. This PR restores those functionalities. It also cleans up some leftover JS code.

Link to relevant issue(s), if any

• Follow-up fix to Issue1347 malihu custom scrollbar fix #1360

Description of the changes in this PR

• select the active node(s) in the hierachy after the page has loaded
• automatically scroll the hierarchy display so that the active node is visible and centered (this is now done using the standard Element.scrollIntoView method instead of using mCustomScrollbar methods)
• remove the getLeafOffset method that was used by the previous scrolling method but is now unnecessary

Known problems or uncertainties in this PR

The auto-scrolling now works a bit differently than in 2.15: it may not scroll exactly the same amount, and the scrolling is smooth instead of instant. I like the smooth scrolling better but this can be easily disabled by adjusting the behavior option of scrollIntoView.

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[codecov]

Codecov Report

Base: 71.17% // Head: 71.17% // No change to project coverage 👍

Coverage data is based on head (2ada973) compared to base (77eb4a2).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #1372   +/-   ##
=========================================
  Coverage     71.17%   71.17%           
  Complexity     1650     1650           
=========================================
  Files            32       32           
  Lines          3788     3788           
=========================================
  Hits           2696     2696           
  Misses         1092     1092           

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[joelit]

Works fine with Chrome, Firefox, and Edge!