The accepted keyboard modifier key for introducing internationalisation features on Windows keyboard mappings is "AltGr". Internally, the AltGr modifier key is equivalent to Ctrl+Alt held down simultaneously. Microsoft advises that applications should not use the Ctrl+Alt or Ctrl+Alt+Shift modifiers for their own purposes, so that these combinations are left clear for keyboard mapping extensions using AltGr.
Unfortunately, Sibelius does employ keyboard short-cuts Ctrl+Alt or Ctrl+Alt+Shift quite extensively. Some of these offending keyboard shortcuts are active during text entry into Sibelius text objects, with the result that text is entered incorrectly.
As an example, on the UK English keyboard, the key combination AltGr+Shift+O is defined to enter the character capital letter Ó acute. But typing that key combination under Sibelius yields small letter ò grave, because Sibelius has a keyboard shortcut defined Ctrl+Alt+Shift+O for ò grave.
To eliminate these clashes, Sibelius should scrap all keyboard shortcuts using modifier keys Ctrl+Alt or Ctrl+Alt+Shift during text entry, leaving these modifier combinations clear for Windows internationalisation and Windows keyboard mapping extensions.