Resize plans properly

[scottbell]

Closes #7587

Describe your changes:

Ensure element isn't null for the resize observer, and that we're deeply watching the size.

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[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 56.17%. Comparing base (4027eae) to head (e4250d3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7597      +/-   ##
==========================================
- Coverage   56.46%   56.17%   -0.30%     
==========================================
  Files         672      672              
  Lines       27112    27111       -1     
  Branches     2635     2635              
==========================================
- Hits        15310    15229      -81     
- Misses      11477    11557      +80     
  Partials      325      325              

Flag	Coverage Δ
e2e-full	23.60% <ø> (-18.12%)	⬇️
e2e-stable	59.93% <ø> (-0.01%)	⬇️
unit	49.13% <66.66%> (+<0.01%)	⬆️

Files	Coverage Δ
src/ui/components/TimeSystemAxis.vue	64.06% <66.66%> (-0.56%)	⬇️

... and 8 files with indirect coverage changes

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[ozyx]

Approved with 1 nitpick that you could take or leave

[ozyx]

I wonder if we can have a visual test that resizes a plan? that might catch a regression like this.