Use different root for renderWhenVisible

[scottbell]

Closes #7405

Describe your changes:

Using the viewport for the IntersectionObserver was causing timing issues when rendering objects on page load. As the application loads itself, the ObjectView can be rendered well off the page, causing the IntersectionObserver to state that the ObjectView is hidden. Strangely, this doesn't appear to be recoverable as I think we're becoming visible through an event that isn't listened to by the IntersectionObserver. Instead, we can use the Open MCT root element as the root object for the IntersectionObserver.

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Author Checklist

• Changes address original issue?
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[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (5c21c34) 55.53% compared to head (c048192) 55.62%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7415      +/-   ##
==========================================
+ Coverage   55.53%   55.62%   +0.08%     
==========================================
  Files         668      668              
  Lines       26701    26704       +3     
  Branches     2585     2585              
==========================================
+ Hits        14828    14853      +25     
+ Misses      11159    11139      -20     
+ Partials      714      712       -2     

Flag	Coverage Δ
e2e-full	41.74% <ø> (-0.01%)	⬇️
e2e-stable	59.50% <75.00%> (+0.38%)	⬆️
unit	48.65% <75.00%> (+0.03%)	⬆️

Files	Coverage Δ
src/plugins/plot/chart/MctChart.vue	43.47% <100.00%> (-0.41%)	⬇️
...ctorDataVisualization/NumericDataInspectorView.vue	0.00% <0.00%> (ø)
src/ui/components/ObjectView.vue	48.13% <50.00%> (ø)
src/utils/visibility/VisibilityObserver.js	90.00% <75.00%> (+0.34%)	⬆️

... and 8 files with indirect coverage changes

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[ozyx]

Not sure how to fix, but I can still reliably reproduce the issue both in e2e test (by running it in a docker container w/ repeatEach: 10), and manually. :/

[ozyx]

LGTM overall, let's just add some comments