Communicate that the open button triggers a sidebar
Usually, if a text button discloses another element, there's an arrow next to the label, like in the case of a dropdown list:
However, we don't do that for the open button in the Text Editor, which makes its behaviour a bit surprising:
Of course, in this case, the sidebar comes from the side rather than appearing below the button, so the standard down arrow approach won't work.
The options I can think of are: 1. add a sidebar icon before the label or 2. replace the label altogether with a sidebar icon.