Minor item since it doesn't affect users, but if you run sigil from the command line so that you see error messages, you get messages like this when switching to CSS or Image tabs since signals are being connected for them when the tab type doesn't support them:
Code:
Warning: Object::connect: No such slot ImageTab::Undo()
Warning: Object::connect: (sender name: 'actionUndo')
Warning: Object::connect: No such slot ImageTab::Redo()
Warning: Object::connect: (sender name: 'actionRedo')
Warning: Object::connect: No such slot ImageTab::Cut()
Warning: Object::connect: (sender name: 'actionCut')
Warning: Object::connect: No such slot ImageTab::Copy()
Warning: Object::connect: (sender name: 'actionCopy')
Warning: Object::connect: No such slot ImageTab::Paste()
Warning: Object::connect: (sender name: 'actionPaste')