searchController: Let an input method process key events

It is expected that users can select a candidate using the keyboard when
an input method shows preedit text and optionally a candidate popup.
To enable this, every key event needs to be propagated if the search
entry has preedit text.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7606

js/ui/searchController.js

@@ -269,6 +269,9 @@ export const SearchController = GObject.registerClass({
     }
 
     _onKeyPress(entry, event) {
+        if (this._text.has_preedit())
+            return Clutter.EVENT_PROPAGATE;
+
         let symbol = event.get_key_symbol();
         if (symbol === Clutter.KEY_Escape) {
             if (this._isActivated()) {