Comment 129 for bug 221112

To sum up

1. French language uses three forms of spaces (normal space, no-break space, short no-break space), as defined in official typographic rules
2. The natural key people expect to output those spaces is the spacebar
3. They can not be put on the first levels because experience shows too many users do not depress shift before the spacebar and generate nobreakspaces accidentally if shift/caps is used as modifier
4. The iso-defined key to access other levels is ctrlgr (right ctrl, see canadian multinational keyboard)

So there is *no* good alternative to use ctrlgr + spacebar to emit those symbols

IIRC xkb has problems distinguishing right ctrl and left ctrl. So you can try to open a bug on this problem so left ctrl + space does not trigger the fifth level.

But right-crtl+space won't be given to apps. If apps expect to grab it, apps are wrong.