title: how to open automatically dropdownbutton when it is focused by focusNode
body: <p>I want to open automatically dropdownbutton when it is focused by focusNode.
  my code just focus dropdownbutton, not open the button.</p>
  <pre><code>Widget _telecomDropdown() {
      return DropdownButtonHideUnderline(
        child: DropdownButtonFormField&lt;String&gt;(
          focusNode: _telecomFocusNode,
          value: _telecomValue,
          isExpanded: true,
          items: telecomItems
              .map((item) =&gt; DropdownMenuItem&lt;String&gt;(
                  value: item,
                  child: Text(
                    item,
                    style: Theme.of(context)
                        .textTheme
                        .titleMedium!
                        .copyWith(fontSize: 16.0.sp),
                  )))
              .toList(),
        ),
      );
    }
  
  TextFormField(
              onChanged: (gender) {
                _isValid();
                if (_isValidGender(gender)) {
                  FocusScope.of(context).requestFocus(_telecomFocusNode);
                }
              },
            ),
  </code></pre>