Hey @bbadmin, thanks for the quick response.
Unfortunately, I tried renaming them in turn each, but even with the newly-created file, the button has disappeared.
To clarify what I'm talking about, on this image (from
https://www.textpad.com/products/textpa ... index.html)
you can see the 'pin' and 'close' buttons in the top-right corner of the Explorer pane on the left-hand-side and in the top-right side of the main window, the 'active files' (down-arrow) and 'close' buttons. In my case (but not in this image), there is also a space for a button on the current active tab, to the right of the tab title text, which you can use to close that tab only.
In my case, those buttons are
completely invisible. Only if I hover my mouse over them (well, over where I know they should be!), can I see the tooltip and a square outline, which I can click to get the action.