Locking and Unlocking XML Markup

For documents with fixed markup, such as forms that do now allow the XML tags to be modified (only their text content), the possibility to edit the XML tag names can be toggled on or off with the Lock / Unlock the XML tags action available in Text editing mode from the Source submenu from the contextual menu (or Document > Source menu).

You can set the default lock state for all opened editors using the Lock the XML tags option in the Text preferences page.