Improve config.sizing_mode behavior

[philippjfr]

Fixes #1059

[codecov]

Codecov Report

Merging #2442 (2bbb3b5) into master (abc56e7) will decrease coverage by 0.01%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##           master    #2442      +/-   ##
==========================================
- Coverage   83.47%   83.46%   -0.02%     
==========================================
  Files         182      182              
  Lines       22668    22674       +6     
==========================================
+ Hits        18923    18924       +1     
- Misses       3745     3750       +5     

Impacted Files	Coverage Δ
panel/viewable.py	68.29% <37.50%> (-1.11%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update abc56e7...2bbb3b5. Read the comment docs.