The Text pane forms the right side the right side of any document window tab, i.e. everything to the right of the vertical spitter bar. By default the pane only shows the Text tab of the text pane (more on the content of the Text tab).
As many Tinderbox projects do not need to export HTML (or any kind of marked-up text) at all, the selectors for these three tabs are hidden by default. To reveal these tab controls Use the Window menu ▸ Shown/Hide Text Pane Selector will toggle the visibility of all sub-tabs.
When first opening the document, if the text pane selector is initially hidden, hiding it is deferred slightly to allow seeing it sliding away, helping to confirm that it is deliberately hidden (and suggesting that there is a way to recover it).
In all three tabs, the text pane's title offers a tooltip containing the path of the note.
The sub-tabs are: