Fix TOC max height size

[MihkelAuv]

Max height of toc-offcanvas is calculated by only subtracting header height from viewport size:

docfx/templates/modern/src/layout.scss

Lines 121 to 127 in a8d1438

	>.toc-offcanvas {
	flex: .35;
	display: block;
	overflow-x: hidden;
	overflow-y: auto;
	max-width: 360px;
	max-height: calc(100vh - #{$header-height});

This causes a portion of TOC to expand out of the viewport if main part of the page isn't scrolled all the way down:

Proposed solution is to also subtract main elements top padding size from viewport height:

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (cf2d134) 77.56% compared to head (c3f9bb7) 77.56%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9035      +/-   ##
==========================================
- Coverage   77.56%   77.56%   -0.01%     
==========================================
  Files         591      591              
  Lines       24577    24577              
==========================================
- Hits        19064    19063       -1     
- Misses       5513     5514       +1     

see 1 file with indirect coverage changes

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[yufeih]

Thank you @MihkelAuv ！