When using a info.magnolia.ui.form.field.definition.SelectFieldDefinition a (Vaadin) filterMode can be defined according to https://documentation.magnolia-cms.com/display/DOCS/Select

This should enable an user to filter the options by a filter-term. This is useful, if you have a lot of options in the select field.

However, this option has no effect because of two reasons:

info.magnolia.ui.form.field.factory.SelectFieldFactory disables text input with setTextInputAllowed(false); on the combobox, which sets "readonly" on the input-field. See https://jira.magnolia-cms.com/browse/MGNLUI-1088

Second reason is that in the same class pageLength is set to 0 (setPageLength(0)). This was done in 5.3.8 (https://jira.magnolia-cms.com/browse/MGNLUI-3297) and disables the previous paging on select fields (which was set to 10).

Problem is, that the vaadin-combobox ignores filters, when pageLength is 0, see

com.vaadin.ui.ComboBox.getOptionsWithFilter(boolean)

The isssue can be solved by re-enabling textinput and set the page-length to a (high) value:

@Override
  protected AbstractSelect createFieldComponent() {
      select = super.createFieldComponent();
      if (select instanceof ComboBox) {
        ((ComboBox) select).setFilteringMode(FilteringMode.CONTAINS);
        ((ComboBox) select).setTextInputAllowed(true);
        ((ComboBox) select).setPageLength(100);
        
      }

      return select;
  }