1. Open a project with whatever unity version that has Overlays feature
2. Create a "Editor" folder in the project and import the attached script in it
3. Make sure the Overlay is visible through the Overlay menu in the scene view (Spacebar shortcut)
4. Right click on the overlay's header and select panel option

You should see all the UI controls having their text displayed as specified by the "text" property in the code.

5. Now, you can try to drag and drop the overlay in the top toolbar or simply change the orientation option to horizontal (Right click header, choose Horizontal option)

Result
The text aren't display if the overlay is not in panel mode

Expect
The Horizontal mode should display the text of the Element like in panel mode

Note that the current attached script is implementing a regular UIElement Button for "Create Cube" element, that why we can see the text.
If we change the Button class to use EditorToolbarButton class instead, the text won't show when docked or in Horizontal orientation.