[Bug] WebRender content becomes blurry at 5120x1440 with NVIDIA eGPU on Linux (X11) - Upstream Firefox has this issue too but not as badly

[tylo]

Preliminary Checks

• I have read and understood the important section above.
• I have searched existing issues and avoided creating duplicates.
• I am not filing an enhancement request.
• I have checked that this issue cannot be reproduced on Mozilla Firefox.
• I have checked that this issue can be reproduced once I removed all my Mods and Custom CSS.

What happened?

When viewing webpages (e.g., reddit, msn) on a high-resolution ultrawide monitor (5120x1440), the entire webpage content becomes blurry and appears downscaled when you stretch the website to be the length of the screen.
Extensive troubleshooting has isolated this issue specifically to Zen Browser. Firefox has the issue as well, but only on very intensive web applications (e.g. aistudio.google.com after logging in)

Google Chrome (Flatpak) running on the exact same hardware and OS do not have this problem.

Comparing about:support logs between Zen and official Firefox reveals that Zen is blocklisting critical hardware acceleration features that Firefox is successfully using.

OS: Pop!_OS 22.04 (X11 Session)
Hardware: NVIDIA RTX 3060 Ti (eGPU setup) via Thunderbolt
Monitor: 5120x1440 @ 120Hz
NVIDIA Driver: 580.82.09
Installation Method: Flatpak

I have performed extensive troubleshooting to isolate the issue:
NVIDIA PowerMizer: Forced "Prefer Maximum Performance" via Xorg config. Issue persists in Zen.
Clean Profile: Performed a full profile reset. Issue persists in Zen.
Overrides: Attempted to disable WebRender via MOZ_WEBRENDER=0 Flatpak env override, but Zen ignores this and force-enables WebRender. (Firefox behaves the same)

Official Firefox build 143.0.3 runs perfectly sharp when visiting webpages like reddit and msn on the same setup. However, visiting an intense web application (e.g. aistudio.google.com after logging in) still produces blurry results.

Zen Browser: Blocklists DMABUF, ACCELERATED_CANVAS2D, and blocks DMABUF_WEBGL.
Official Firefox: Shows DMABUF, ACCELERATED_CANVAS2D, and CANVAS_RENDERER_THREAD as "available".

Expected behavior

The web application should remain sharp and render at the native resolution, regardless of window size.

Actual behavior

When the window exceeds a certain size threshold, the rendering quality drops significantly, resulting in a blurry, upscaled image.

Steps to reproduce

Launch Zen Browser (Flatpak) on Pop!_OS (X11 session) with an NVIDIA eGPU connected to a 5120x1440 monitor.
Ensure hardware acceleration is enabled (WebRender is active).
Navigate to a website (e.g., reddit, msn).
Maximize the window across the full 5120x1440 display.

Screenshots and videos

zen-support.json

firefox-support.json

Version

1.16.4b

What platform are you seeing the problem on?

Linux (Flatpak)

What component is this issue related to?

Performance

Relevant log output if applicable

[tylo]

Also submitted a bug to Mozilla, but found it interesting that Zen was having even worse performance problems with this blur issue so decided to also bring it up here.

https://bugzilla.mozilla.org/show_bug.cgi?id=1993838