Ensure Tab headers do not scroll unnecessarily

[philippjfr]

The width of the tab headers was incorrectly calculated leading to a scroll bar appearing when it wasn't needed.

[codecov]

Codecov Report

Merging #4146 (d849352) into master (c1e14aa) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4146      +/-   ##
==========================================
- Coverage   37.62%   37.56%   -0.07%     
==========================================
  Files         224      224              
  Lines       33382    33382              
==========================================
- Hits        12561    12539      -22     
- Misses      20821    20843      +22     

Flag	Coverage Δ
ui-tests	37.56% <ø> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
panel/tests/ui/io/test_jupyter_server_extension.py	50.00% <0.00%> (-50.00%)	⬇️
panel/tests/ui/io/test_jupyterlite.py	83.78% <0.00%> (-8.11%)	⬇️
panel/tests/ui/io/test_convert.py	90.26% <0.00%> (-2.66%)	⬇️
panel/tests/conftest.py	50.86% <0.00%> (-1.30%)	⬇️
panel/tests/util.py	55.88% <0.00%> (-0.99%)	⬇️
panel/io/cache.py	23.21% <0.00%> (ø)
panel/io/state.py	34.30% <0.00%> (+0.18%)	⬆️
panel/io/server.py	29.71% <0.00%> (+0.18%)	⬆️

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