Customize menu > Lock UI Layout
When the Lock Interface Layout toggle is active, you cannot modify the user interface layout. Use this command to prevent inadvertent user interface changes.
Tip: Use this command if you find you are inadvertently selecting and floating UI elements such as the Command panel.
Procedure
To lock the UI layout:
Choose Lock UI Layout from the Customize menu.
You can no longer move or resize the UI elements. Use this command to prevent user interface changes or if you find yourself accidentally floating toolbars or the command panels on mouse clicks.