Blurry content on a 4K monitor on Windows 10

[Luuk3333]

Attach (recommended) or Link to PDF file here:
https://mozilla.github.io/pdf.js/web/viewer.html

Configuration:

• Web browser and its version: Firefox version 66.0.5
• Operating system and its version: Windows 10 Pro, version 10.0.17763 Build 17763
• PDF.js version: 2.2.167
• Is a browser extension: No

Steps to reproduce the problem:

1. Attach a 4K monitor to a PC running Windows 10
2. Change the Firefox preference layout.css.devPixelsPerPx to a value higher than '1'.
3. Open https://mozilla.github.io/pdf.js/web/viewer.html or a local PDF.

What is the expected behavior?

The content of the PDF should be scaled according to the preference listed above.

What went wrong?

pdf.js displays PDFs blurry when using a 4K monitor on Windows 10. The only preference I have changed to get Firefox set up for a 4K monitor is the preference layout.css.devPixelsPerPx which I've set to '1.9' resulting in Firefox nicely scaling both Firefox and websites on the 27 inch monitor. However, the more that preference is increased the more blurry the built-in viewer displays PDFs. All content of PDFs is blurry, not just text. Also, zooming does not fix the problem.

Here's a comparison between Firefox, Chrome and Edge with the latter two displaying the content correctly:

[brendandahl]

If you leave the pref layout.css.devPixelsPerPx as the default, does the text look okay?

[Luuk3333]

@brendandahl No, still blurry unfortunately :/. The default is '-1.0', which on a 4k monitor is the equivalent of '2'.
Here's a screenshot with preference set to the default:

[Luuk3333]

Firefox is reporting an incorrect devicePixelRatio of '1'. Forcing it to be '2' by setting the variable devicePixelRatio on line 95 of the file 'web/ui_utils.js' to '2' fixes the issue as the content is now sharp. This is not a pdf.js issue but rather a Firefox issue.

[brendandahl]

@Luuk3333 Did you file a bug upstream with Firefox? https://bugzilla.mozilla.org

[Snuffleupagus]

This may be related to, or even a duplicate of, https://bugzilla.mozilla.org/show_bug.cgi?id=1537955.

[Luuk3333]

@Luuk3333 Did you file a bug upstream with Firefox? https://bugzilla.mozilla.org

I have not. However, Firefox seems to be reporting the correct device pixel ratio since version 67.0.

[tannenfreund87]

This may be related to, or even a duplicate of, https://bugzilla.mozilla.org/show_bug.cgi?id=1537955.

To connect the issues better, here is the appropriate Firefox bugreport: https://bugzilla.mozilla.org/show_bug.cgi?id=1771184