A select's selected option renders as the first option in poppler

[jakejackson1]

Summary

A <select> with a selected option, rendered with useActiveForms, shows its first option in poppler rather than the selected one. mPDF writes the field correctly as far as I can tell: /Opt holds export-and-label pairs and /V and /DV hold the export value of the selected option. What needs settling is whether /V should hold the export value or the label, since poppler evidently matches it against the label, and what Acrobat and Chrome show.

Found while building the forms snapshot document for #38, which renders a select in each of its two blocks; the rasters show "First option" although the second is marked selected. Same on gravitypdf.

Try it

$mpdf = new \Mpdf\Mpdf();
$mpdf->useActiveForms = true;
$mpdf->WriteHTML('<select name="s">
  <option value="1">First option</option>
  <option value="2" selected="selected">Second option</option>
</select>');
$mpdf->Output('select.pdf', 'F');

The field object carries /Opt [ [(1) (First option)] [(2) (Second option)] ] /V (2) /DV (2). pdftoppm renders "First option" in the box.

Where to look

• ISO 32000-1 §12.7.4.4 on choice fields, for whether /V is the export value or the display string when /Opt uses pairs.
• Form::_putform_ch() writes /V from the option's value attribute.
• Acrobat and Chrome, for what they display for the same file; if both show the selected option, this is a poppler difference and the issue can be closed with a note.

Test plan

• A unit case on the emitted document asserting /V for a selected option, in whichever form the spec settles on, and one for a select with no selected option.
• The forms snapshot document from Make page-break-inside: avoid dependable #38 re-rendered; its two selects should read "Second option, selected" in the raster if the fix reaches poppler's matching.