Tested with:
Nightly 91.0a1 (2021-06-10)

Tested on:
Win 10

Preconditions:
In about:config, set pdfjs.enableXfa = true

Steps:

1. Launch firefox
2. Open the following pdf: https://bug1671648.bmoattachments.org/attachment.cgi?id=9182033
3. Click on "+" icon to zoom in the pdf

Actual result:
Zoom is not applied on dropdown list items and the text is blurry. See screenshot

Expected result:
Zoom should be applied in dropdown list items as in Adobe Reader.

We have a similar bug for AcroForms.

This issue also occurs on Mac OS 11.4 and Ubuntu 20.04.

Should this be blocking the [meta] JS support for XFA? (bug 1717661)

(In reply to Bodea Daniel [:danibodea] from comment #3)

This issue also occurs on Mac OS 11.4 and Ubuntu 20.04.

Should this be blocking the [meta] JS support for XFA? (bug 1717661)

Sorry, missed the question. No it isn't related to JS, but it is a generic PDF.js bug that does not only affect XFA.
We already have a bug on file about this, but I can't find it now.

This is not XFA specific, but affecting AcroForms too (e.g. you can reproduce it on https://tcpdf.org/files/examples/example_054.pdf).

In pdf.js, zoom is made thanks to transform: matrix(....) on the different html elements.
The transform has an effect on the size and the font size of the select button but not on the select content.

A way to achieve this is to have a css variable to represent the scale factor which will be updated using js.
And then we can have something like:

transform: matrix(var(--factor), 0, 0, var(--factor), 0, 0);
font-size: calc(10pt * var(--factor));

Moving to S2 given the a11y implications.

This fix has been verified using the original reproduction steps + an acrofom test page. The fix is confirmed on Nightly v93.0a1 from 2021-08-15 and Beta v92.0b4, while it is still reproducing in Release v91.0, but disabled under pref (in the case of XFA, not acroform). Tested in Windows 10.