(Not updated for a long time. May be broken)

syntax-highlighter

Red syntax/expressions highlighting

Usage and limitations

Expressions highlighting works by default for predefined functions only. If UDF is evaluated in step mode, highlighting will work on these too (They are evaluated in default context; in-object evaluation TBD)
op! scope is correctly found when treated as argument. Arguments highlighting for op! itself (in "Expr" mode) is partly done 17.01 (Left nested parens TBD)
Highlighting does not adapt to dialects (e.g. parse, VID, RTD). TBD
Editing:

First draft done 18.01: In "Step" mode use "Eval" button to evaluate highlighted expression
Refining 7.02: Click "Construct". Then in step mode select expression and ctrl-click it to open refining panel... Works only for programs not in anonymous or path-named contexts.

By scroller (from start)
Simple wheeling done 18.01
Back-stepping added 19.01
Step-selection by mouse click 19.01
Simple search added 20.01. Pointing on element, choose "Show" from contextual menu. For navigation between highlighted elements, select "Next" or "Prev", use contextual menu or arrow-keys.
"Find" added to contextual menu 21.01. Next/prev find with buttons, contextual menu or arrow-keys.
Move "Step"-highlighted elements: buttons, menu and keys (down-arrow-key = "into" while stepping, enter = evaluate)

Layout is done according to W10. It may show with defects on other platforms/versions.
Hovering worked initially for ~1.5 page only.

Using bold in syntax highlighting style definitions has no effect on caret-to-offset calculations, which causes misplacement of hover-reactive boxes on layer above rich-text. Red bug?