the ComboBox widget fails to recognise CTRL + <any non-special key> (e.g. CTRL+A, CTRL+Z, CTRL+Y, CTRL+C etc) as a special key combination, and fires off a query anyway (test-case:, simply because CTRL and <any non-special key> are handled as separate keyup events.

the following override resolves this issue:
Ext.override(Ext.form.ComboBox, {
    onKeyUp : function(e) {
        if (this.editable !== false 
                && !e.isSpecialKey() // existing code -- correctly recognises special keys
                && (!e.hasModifier() || e.shiftKey) // only allow the shift key modifier
        ) {
            this.lastKey = e.getKey();