Fix handling of split MenuButton clicks

[philippjfr]

Fixes #4020

[codecov]

Codecov Report

Merging #4021 (000f36e) into master (9c819d0) will increase coverage by 9.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4021      +/-   ##
==========================================
+ Coverage   74.65%   83.66%   +9.01%     
==========================================
  Files         223      223              
  Lines       33148    33152       +4     
==========================================
+ Hits        24748    27738    +2990     
+ Misses       8400     5414    -2986     

Flag	Coverage Δ
ui-tests	37.60% <25.00%> (?)
unitexamples-tests	74.66% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
panel/widgets/button.py	89.74% <100.00%> (+0.55%)	⬆️
panel/param.py	87.22% <0.00%> (+0.15%)	⬆️
panel/io/state.py	70.05% <0.00%> (+0.18%)	⬆️
panel/pane/base.py	87.91% <0.00%> (+0.41%)	⬆️
panel/pane/image.py	79.53% <0.00%> (+0.46%)	⬆️
panel/widgets/slider.py	95.14% <0.00%> (+0.74%)	⬆️
panel/reactive.py	78.09% <0.00%> (+0.74%)	⬆️
panel/widgets/tables.py	87.85% <0.00%> (+1.07%)	⬆️
panel/tests/pane/test_vtk.py	100.00% <0.00%> (+1.48%)	⬆️
panel/tests/pane/test_deckgl.py	100.00% <0.00%> (+1.85%)	⬆️
... and 29 more

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[xnsde]

As the author of the issue #4020, I can confirm it fixes it for me. Thanks for the quick turnaround!