I have written a javascript-generated web page which can be opened from a lesson page to provide an interactive flashcard exercise for students of Chinese. It functions on a (now deprecated!) frameset page with three 'columns' having frame pages the second and third of which are written on the fly by javascript in the first frame page. Each of the second and third frame pages has an html select dropdown, written with identically structured code. However, although the dropdown in the third frame page appears with a vertical scrollbar, the one in the second frame page (a) has no scrollbar, and (b) 'drops up' (in Firefox on a Linux platform) to extend beyond the top edge of the main browser window, making the top of the dropdown list inaccessible. The structure of the html form containing the select tag is:
<form>
<select size="1" name="sel_py" id="sel_py">
<option class="lucidasansu" value="">CHOOSE</option>
<option class="lucidasansu" value="er4">èr</option>
<option class="lucidasansu" value="hang2">háng</option>
<option class="lucidasansu" value="hao3">hǎo</option>
....
<option class="lucidasansu" value="ye3">yě</option>
<option class="lucidasansu" value="yi1">yī </option>
</select>
<span id="py_butt"><input type="button" name="py_button" value="CHECK" onClick="test_py(this.form.sel_py.value)"></span>
</form>
The tag has no 'action' value because the effect of the onClick on the button identified as py_button works within the group of frame pages to check the validity of the choice and to reveal a similar button on the third frame page once the first selection has succeeded.
I have checked out the code of both the second and the third frame pages on the W3C markup validator and they check out with only minor warnings, so I am at a loss to see why two apparently identically-structured dropdowns should have such different performances.