Testing SDL_TextEditingEvent

From the documentation I couldn’t figure out what SDL_TextEditingEvent::start is for.
I tried composing text segments in Hiragana but start always stays at 0.
Is there an input method where start of TextEditingEvent/SDL_TextEditingExtEvent changes, so I could test its behavior? This question applies to Linux and Windows.
Furthermore is there a way to query the composition’s cursor position in SDL? GTK+ applications seem to be able to display this but SDL pushes no events as to when the cursor moves within the current composition.