Change CSS for help icon so that when scroll or truncate is used, it is not clipped. (mathjax/MathJax#3432)

[dpvc]

This PR update the CSS used for the visual help icon when the explorer is running so that it:

• doesn't get clipped when used with overflow="scroll" or overflow="truncate".
• floats at the right during scrolling for overflow="scroll" and isn't truncated for overflow="truncate"
• puts the icon next to a displayed expression when there isn't an equation tag (rather than at the far right as currently).

Resolves issue mathjax/MathJax#3432.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.71%. Comparing base (afaf1ea) to head (654924f).
⚠️ Report is 67 commits behind head on develop.

Files with missing lines	Patch %	Lines
ts/output/common.ts	50.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1350      +/-   ##
===========================================
- Coverage    86.72%   86.71%   -0.01%     
===========================================
  Files          337      337              
  Lines        84145    84146       +1     
  Branches      3140     4769    +1629     
===========================================
  Hits         72971    72971              
+ Misses       11174    11152      -22     
- Partials         0       23      +23     

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[zorkow]

• puts the icon next to a displayed expression when there isn't an equation tag (rather than at the far right as currently).

I find this too close to the expression; see below. Can we somehow have it on the edge of the top-level highlighter box?

[dpvc]

This is the same positioning use for inline expressions. The overlap is intentional to help link the info icon to the expression itself. I suppose it could be moved to the right for display expressions, but right now it is consistent with the inline placement.

[zorkow]

lgtm.