Improve textviewer rendering speed

[david-yz-liu]

Proposed Changes

(Describe your changes here. Also describe the motivation for your changes: what problem do they solve, or how do they improve the application or codebase? If this pull request fixes an open issue, use a keyword to link this pull request to the issue.)

When the text viewer renders large files, the run_syntax_highlighter function currently takes a significant amount of time, and causes the page to freeze. A user reported this after uploading a CSV file with over 7000 lines.

The issue was not with Prism.js (the syntax highlighting library we're using), but rather with the code that was modifying the result of the syntax highlighting to make it compatible with our own source code glower. I refactored this code to improve its performance, mostly by making better use of Node/Element methods.

Screenshots of your changes (if applicable)

Associated documentation repository pull request (if applicable)

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)
🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)	X
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
🔧 Internal (change that only affects developers or continuous integration)

Checklist

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Before opening your pull request:

• I have performed a self-review of my changes.
  • Check that all changed files included in this pull request are intentional changes.
  • Check that all changes are relevant to the purpose of this pull request, as described above.
• I have added tests for my changes, if applicable.
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Questions and Comments

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[coveralls]

Pull Request Test Coverage Report for Build 10694123344

Details

• 17 of 17 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 91.488%

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Totals
Change from base Build 10656295102:	0.04%
Covered Lines:	40237
Relevant Lines:	43310

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💛 - Coveralls