* section labels, TOC, speedups and fixes
* show section numbers in widgets
* debounce livePreview in usercss editor
* better fixed header and compact layout compatibility
* fix section sizing for compact layout + layout speedup
* DocFuncMapper + cosmetics + fix Clone button
* don't run linter during initSections
* remove unused/unnecessary DOM polyfills
* report invalid @document function as parser error
* rewrite section finder
* simplify focusedViaClick
* simplify setPreprocessor and make it synchronous
* throttle offscreen line widgets in usercss with lots of sections
* add on, off aliases for add/removeEventListener + onOff
* use on/off aliases in changed files
* use getters in more places
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