I suggest to make it a member function of EventData.

Space after if.

Also, please use {} braces even for single-line statements after if. Needs to be fixed in multiple places.

Why static? Doesn't it always need to access the xkb and dpy from the instance?

most probably this can be a QHash, as there is no requirement to keep the elements sorted by the key

this method can be 'const', as it does not change EventData

this can do return keycode; directly

the code out of the loop can be a simple return NoSymbol

const_iterator + constFind

constEnd

use insert() directly, which is slightly faster than creating an element and changing it immediately after

use take(), do avoid doing a lookup twice