This bug is still present with the PDF.js viewer in Version 0.99.128.2.1-beta (WebKit 619.1.11.111.2). Here are the steps to reproduce this behaviour with a clean profile:
- Open a clean Orion profile.
- Enable the PDF.js viewer under the Develop > Experimental Settings (if disabled)
- Load a pdf file like https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf
- Note that only the PDF control buttons appear, but the file itself isn't shown.
- Reload the page to see the PDF file itself.
This behaviour is not present with the default WebKit PDF viewer.
Here is a video of the bug being reproduced in a clean Orion profile: