I select some text in document editor window, and it is highlighted.
I select a different document in the binder, which then appears in the editor.
I go back to the first document. The text is still highlighted, but it does not seem to be actually selected, because pressing Delete, or ctrl C etc has no effect.
When you say you “go back to the first document”, what method are you using to arrive there? For instance, if I press Ctrl-[ or click the back arrow button above the editor to navigate back in history, then my selection is active because I have never moved the keyboard focus out of the editor. If, however, I click on a different document in the binder sidebar then I can see my selection, but I’ve clicked in the sidebar, not the editor. I can’t be typing and editing as my keyboard is “pointed at” the sidebar. You can use the up and down arrow keys in the sidebar to see how this is so. To get back to the editor without clicking (and destroying the selection), use Ctrl-Tab. This switches between editor and binder (and the other editor as well, when you have the editor split into two).
Thanks - my mistake!