Fix Perspective drag errors

[MarcSkovMadsen]

Fix #2299

Added tests and a fix.

[codecov]

Codecov Report

Merging #2300 (df0e078) into master (078c8b2) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2300      +/-   ##
==========================================
+ Coverage   83.68%   83.69%   +0.01%     
==========================================
  Files         183      183              
  Lines       22018    22034      +16     
==========================================
+ Hits        18425    18442      +17     
+ Misses       3593     3592       -1     

Impacted Files	Coverage Δ
panel/pane/perspective.py	54.14% <100.00%> (+0.43%)	⬆️
panel/tests/pane/test_perspective.py	100.00% <100.00%> (ø)

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