RichTextFX

https://github.com/FXMisc/RichTextFX 

https://github.com/FXMisc/RichTextFX#who-uses-richtextfx

https://github.com/gaeqs/JAMS

Who uses RichTextFX?

    Alt.Text
    Arduino Harp
    Astro IDE
    BasicCAT
    BlueJ
    Boomega
    Chorus
    Chronos IDE
    George
    Greenfoot
    EpubFx
    Everest REST client
    JabRef
    JDialogue
    JuliarFuture
    Kappa IDE
    KeenWrite
    Markdown Writer FX
    mqtt-spy
    Nearde IDE
    OmniEditor
    PsychSQL
    SqlBrowserFx
    Squirrel SQL client
    Xanthic
    XR3Player

Let us know if you use RichTextFX in your project!
Features

    Assign arbitrary styles to arbitrary ranges of text. A style can be an object, a CSS string, or a style class string.
    Display line numbers or, more generally, any graphic in front of each paragraph. Can be used to show breakpoint toggles on each line of code.
    Support for displaying other Nodes in-line.
    Positioning a popup window relative to the caret or selection. Useful e.g. to position an autocompletion box.
    Getting the character index under the mouse when the mouse stays still over the text for a specified period of time. Useful for displaying tooltips depending on the word under the mouse.
    Overriding the default behavior only where necessary without overriding any other part.