From v6.0, the text uses essentially the same methodology for RTF layout that is seen in TextEdit. The old $LeftMargin and $Right margin exist in (old(er)) TBX documents only for use with pre v6 Tinderbox.
By default, Tinderbox $text in a new note has a fixed left and un-set auto-wrapping right margin. However, if you copy/paste from an RTF source that does has a fixed page width/margin, the pasted data will respect the source's** formatting and may result in a fixed note text width and/or right margin. Each paragraph in v6 $Text
may have a discrete style (font, width, tabs, etc.). The cocoa framework's supplied management for this is pretty lousy and poorly documented.
The easiest want to spot such non-default page style is to turn on the note's text pane ruler ([Opt]+[Ctrl]+R) and click in each paragraph noting margin/tab settings.
If you have odd margins like this as a result of pasting, select the $Text (or affected sections within it) and use menu Format -> Style -> Reset Margins.
** To strip all such formatting in a paste, use 'Paste and Match Style' ([Cmd]+[Opt]+[Shift]+V). This results in all formatting (colour, fonts, margins, tabs, line spacing, etc.) being stripped. Only the text characters are pasted (including white space and line breaks).
Pro tip: when pasting into Tinderbox from other apps always use Paste and Match Style unless you definitely know you want any source format. It's often easier/quicker to stick back in a bold heading than it is spending hours trying to fix imported non-matching paragraph styles (for which the controls - derived from the Apple frameworks they use - are not very good IMO). The latter's not Eastgate's fault, the frameworks control what's possible and, in the round, aide lots of other good things unrelated to margins and printing: it's a matter of balance. So, if importing, try to get in the habit of using Paste and Match Style!
Edit: I've updated the aTbRef page '
Working with $Text: support for styled text'. Remember in most cases you probably want 'Paste and Match Style' instead of 'Paste' when importing text via a copy/paste