Bug 489464

Summary:	Fractionally-scaled single-pixel lines sometimes become 2px when their window is moved or resized
Product:	[Plasma] kwin	Reporter:	Edan <eddielaz.p>
Component:	wayland-generic	Assignee:	KWin default assignee <kwin-bugs-null>
Status:	REPORTED ---
Severity:	normal	CC:	kik0swd, nate
Priority:	NOR	Keywords:	wayland
Version:	6.1.2
Target Milestone:	---
Platform:	Arch Linux
OS:	Linux
Latest Commit:		Version Fixed In:
Sentry Crash Report:

Description Edan 2024-06-29 23:40:27 UTC

Problem:
Fractional Scaling on High DPI screens causing visual inconsistency.

Steps to reproduce:
1. First off you need a monitor considered high dpi, e.g. 4k
2. Set scaling to fractional, this means values like: 125%, 150%, 175%... The bug doesn't happen if the scaling is 100%, 200% etc.
3. Now in certain borders, applications, when resizing or moving the app, sometimes the borders that supposed to be 1px turns out to be 2px, the look and the feedback of this feels very, bad and btw I tested this on others Linux distro and the issue persist, I'm not sure if is a qt issue, or KDE issue, another thing I'm sure is that this happens in both Qt Quick and Qt Widgets applications

Additional Notes:
I think the issue is some algorithm/calculation which is not 100% correctly, dealing in Qt/KDE apps to render wrong pixels.

Expected Result:
Maintain visual fidelity for fractional scaling.

Software/OS Versions:
Linux/KDE Plasma: Arch Linux

Comment 1 MemoryLeak 2024-06-30 00:09:12 UTC

I can confirm this since it is happening to me.

Comment 2 Nate Graham 2024-07-02 20:45:30 UTC

There are multiple issues we're aware of and are working to fix. However there's no way of knowing which specific issue this is without a screenshot. Can you attach one?

Also, you're using Wayland, right?

Comment 3 Edan 2024-07-06 06:04:57 UTC

(In reply to Nate Graham from comment #2)
> There are multiple issues we're aware of and are working to fix. However
> there's no way of knowing which specific issue this is without a screenshot.
> Can you attach one?
> 
> Also, you're using Wayland, right?

Sorry for the delay to answer I've been busy, I did a Youtube video which captures all the Qt issues that I'm talking about here, 

https://www.youtube.com/watch?v=OclGA2-dr2g

Is not an OBS mistake or bug, is just how it looks natively, the video begins showing on windows, but at second 00:25 and later I show under Linux. Yes I'm using Wayland, but this issue take place in every Linux when fractional scaling take place, this is a Qt issue since this happens everywhere when you meet the requirements to reproduce it, 
1- Having a high dpi monitor >= 4k.
2- Set scaling to fractional. 
I don't dispose of a mac at the moment so I'm not sure how Qt is behaving there, but I think is less noticeable there, since mac uses fixed resolutions and scaling. Maybe it does happen there too, I'm not sure.

Comment 4 Bug Janitor Service 2024-07-21 03:46:21 UTC

Dear Bug Submitter,

This bug has been in NEEDSINFO status with no change for at least
15 days. Please provide the requested information as soon as
possible and set the bug status as REPORTED. Due to regular bug
tracker maintenance, if the bug is still in NEEDSINFO status with
no change in 30 days the bug will be closed as RESOLVED > WORKSFORME
due to lack of needed information.

For more information about our bug triaging procedures please read the
wiki located here:
https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

If you have already provided the requested information, please
mark the bug as REPORTED so that the KDE team knows that the bug is
ready to be confirmed.

Thank you for helping us make KDE software even better for everyone!

Comment 5 Nate Graham 2024-07-23 18:35:40 UTC

The text issue is https://bugs.kde.org/show_bug.cgi?id=479891, which is ultimately a Qt bug.

The issue with single-pixel lines is likely a Qt issue as well, but I'll let the KWin folks decide. Moving to KWin.