Locking and Unlocking XML Markup

For documents with fixed markup such as forms in which the XML tags are not allowed to be modified but only their text content, editing of the XML tag names can be disabled and re-enabled with the action available from Document > Source > Locks / Unlocks the XML Tags or from the toolbar button Locks / Unlocks the XML tags

There is a default lock state for all opened editors in the Preferences XML Editor Format page.