By default Glean will show your audio feed as you record. You can hide this using the Layout options.
Use the Layout options on the top right to hide this pane when recording
As you resize your Glean window the layout automatically changes to give you the most valuable items on screen.
As the screen size reduces the Audio feed gets hidden - this happens whether you have the above setting enabled or not.
As the size reduces further we shift the panels, so your text feed is moved underneath your slides.