[BUG] Background image that should be blocked by uBO is appearing

Bug Report

Describe the bug

I honestly am not sure at this point what is going on here. On the surface, it appears that Dark Reader is showing an image that should have been blocked by uBlock Origin. This needs more investigation to determine what is going on.

I’ve seen this issue before, but it was not easy to replicate. Finally, I’ve found a place where I can easily replicate it. See STR.

Steps to reproduce

1. Install uBlock Origin (uBO) v1.34.0 in Firefox
2. Install Dark Reader v4.9.31 in Firefox
3. Enable Dark Reader v5 beta mode in v4.9.31 and be sure Dynamic dark mode is enabled
4. Go to https://sourceforge.net/
5. Notice the background image that looks like this:
6. Add uBO filter: ||fsdn.com/con/$important and save filters in uBO
7. Go back to the Firefox tab with https://sourceforge.net/
8. Hold down the Shift key, and press Firefox’s refresh button to refresh the page while bypassing the browser’s cache
9. Notice that the background mentioned above is still present
10. Use Dark Reader’s drop-down menu to disable Dark Reader
11. Notice that the background mentioned above disappears
12. Use Dark Reader’s drop-down menu to re-enable Dark Reader
13. Notice that the background mentioned above re-appears

Expected behavior

The background image to no longer appear regardless of whether or not Dark Reader is enabled.

Actual behavior

The background image only appears when Dark Reader is enabled.

Screenshots

System Information

• OS: Windows
• Browser: Firefox 87.0
• Dark Reader Version: v4.9.31

Additional context

Normally, this would just be a simple case of using the Dynamic Theme Editor to tweak the dynamic theme. But in this case, I don’t currently understand why the background image is being shown at all given that it should be blocked by uBO. It’s like Dark Reader is bypassing uBO filtering, but this requires further investigation before that conclusion can be made.

[Gusted]

Dark Reader could use a listener on webRequest.onErrorOccurred to find out which resources should not be fetched.

Ah, thanks for the suggestion. Will have a PR for this up soon.

[gorhill]

Is there any way to get around Dark Reader calling resources that are blocked by other extensions (such as uBO)?

Not that I know of.

Dark Reader could use a listener on webRequest.onErrorOccurred to find out which resources should not be fetched.

Regardless of uBO, it would be best that DR respects the blocking of resources from content blockers.