Ensure FloatPanel reflects closed status

[philippjfr]

Fixes #5438

[codecov]

Codecov Report

Merging #5466 (dc04828) into main (92e3982) will decrease coverage by 0.05%.
The diff coverage is 46.15%.

@@            Coverage Diff             @@
##             main    #5466      +/-   ##
==========================================
- Coverage   72.57%   72.53%   -0.05%     
==========================================
  Files         274      275       +1     
  Lines       40111    40124      +13     
==========================================
- Hits        29109    29102       -7     
- Misses      11002    11022      +20     

Flag	Coverage Δ
unitexamples-tests	72.53% <46.15%> (-0.05%)	⬇️

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Files Changed	Coverage Δ
panel/layout/float.py	85.71% <ø> (ø)
panel/tests/ui/layout/test_floatpanel.py	46.15% <46.15%> (ø)

... and 6 files with indirect coverage changes

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