improve getBoundingClientRect in mockdom

[nightwing]

Generic mockdom improvements that were needed for #5922

This also revealed that one of autocomplete tests is broken and was passing only in mockdom.

Not sure if we need to change the test, or update the behavior of the popup to match the test description.

Open kitchen-sink @ f687475194a849d8def1e9edd5a9e0c85d2c635a

[codecov]

Codecov Report

❌ Patch coverage is 92.09302% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.73%. Comparing base (c89c0d5) to head (f687475).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/test/mockdom.js	87.81%	34 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5973      +/-   ##
==========================================
- Coverage   93.73%   93.73%   -0.01%     
==========================================
  Files         640      640              
  Lines      137743   138063     +320     
  Branches    14487    14538      +51     
==========================================
+ Hits       129113   129412     +299     
- Misses       8630     8651      +21     

Flag	Coverage Δ
unittests	93.73% <92.09%> (-0.01%)	⬇️

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