Ext.CheckboxSelectionModel.override({
    handleMouseDown : function(g, rowIndex, e){
        if(e.button !== 0 || this.isLocked()){
            return;
        };
        var view = this.grid.getView();
        if(e.shiftKey && this.last !== false){
            var last = this.last;
            this.selectRange(last, rowIndex, e.ctrlKey);
            this.last = last; // reset the last
            view.focusRow(rowIndex);
        }else{
            var isSelected = this.isSelected(rowIndex);
            if(e.ctrlKey && isSelected){
                this.deselectRow(rowIndex);
            }else if(!isSelected || this.getCount() > 1){
                this.selectRow(rowIndex, true);
                view.focusRow(rowIndex);
            }
        }
    }
});

Extjs CheckboxSelectionModel keepExisting

JavaScript