I tried the following, but nothing helped:
- disabling anti-aliasing
- changing sub-pixel rendering
- changing hinting
- changing scaling method for legacy applications (X11)
- changing RGB range
- changing font face
- changing QT_AUTO_SCREEN_SCALE_FACTOR, QT_ENABLE_HIGHDPI_SCALING, QT_SCALE_FACTOR, QT_SCREEN_SCALE_FACTORS, QT_FONT_DPI

Below is the output of some commands, maybe it will be helpful.

> $ xdpyinfo | grep -B 2 resolution
> screen #0:
>   dimensions:    1920x1080 pixels (508x285 millimeters)
>   resolution:    96x96 dots per inch

> $ xrdb -query
> Xcursor.size:   30
> Xcursor.theme:  breeze_cursors
> Xft.antialias:  1
> Xft.dpi:        120
> Xft.hinting:    1
> Xft.hintstyle:  hintslight
> Xft.rgba:       rgb

> $ xrandr -q
> Screen 0: minimum 16 x 16, current 1920 x 1080, maximum 32767 x 32767
> eDP-1 connected primary 1920x1080+0+0 (normal left inverted right x axis y axis) 340mm x 190mm
>    1920x1080     59.96*+
>    1440x1080     59.99
>    1400x1050     59.98
>    1280x1024     59.89
>    1280x960      59.94
>    1152x864      59.96
>    1024x768      59.92
>    800x600       59.86
>    640x480       59.38
>    320x240       59.52
>    1680x1050     59.95
>    1440x900      59.89
>    1280x800      59.81
>    1152x720      59.97
>    960x600       59.96
>    928x580       59.88
>    800x500       59.50
>    768x480       59.90
>    720x480       59.71
>    640x400       59.95
>    320x200       58.96
>    1600x900      59.95
>    1368x768      59.88
>    1280x720      59.86
>    1024x576      59.90
>    864x486       59.92
>    720x400       59.55
>    640x350       59.77

Is that dolphin a Qt6 build?

Can you run:

 WAYLAND_DEBUG=1 dolphin |& grep fractional

Yes, it's Qt6 + Wayland.

> $ WAYLAND_DEBUG=1 dolphin |& grep fractional
> [2710477.444] wl_registry@2.global(8, "wp_fractional_scale_manager_v1", 1)
> [2710477.447]  -> wl_registry@2.bind(8, "wp_fractional_scale_manager_v1", 1, new id [unknown]@7)
> [2710495.844] wl_registry@35.global(8, "wp_fractional_scale_manager_v1", 1)
> [2710694.078]  -> wp_fractional_scale_manager_v1@7.get_fractional_scale(new id > wp_fractional_scale_v1@45, wl_surface@44)
> [2710743.771] wp_fractional_scale_v1@45.preferred_scale(150)
> [2710973.623] wl_registry@2.global(8, "wp_fractional_scale_manager_v1", 1)
> [2710973.630]  -> wl_registry@2.bind(8, "wp_fractional_scale_manager_v1", 1, new id [unknown]@7)
> [2711034.716] wl_registry@35.global(8, "wp_fractional_scale_manager_v1", 1)
> [2718406.390]  -> wp_fractional_scale_v1@45.destroy()

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!

What other requested information can I provide to you?

You did fine; David just needs to go over the logging you provided now.

I've got similar issues after upgrading to Fedora 40 and Plasma 6. 
A global scaling of 125% creates all sorts of issues with fonts. Fonts look blurry indeed. 

It's especially obvious when using Kmail/Kontact. Email content looks terrible with apparently pixelated, non anti-aliased fonts => maybe this needs its own bug report? 

A scaling of 100% fixes all of it, but makes the general interface way too small for fairly high DPI screens. 

System details:
Fedora 40
Kernel version: 6.8.8-300.fc40.x86_64
KDE Plasma: 6.0.4
KDE Frameworks: 6.1.0
Qt version: 6.7.0
Graphics platform: Wayland

@popov895 @Debby are you by chance using Lightlyshaders, KDE-Rounded-Corners, Colorblindness Correction, Invert or any such kwin effect? Seemingly, kwin doesn't handle them properly when using fractional scaling on wayland.

Possibly related: https://bugs.kde.org/show_bug.cgi?id=482978

(In reply to Stefan Springer from comment #8)
> Lightlyshaders, KDE-Rounded-Corners, Colorblindness Correction, Invert

I don't use any of these effects, only the ones that are enabled by default. Moreover, turning off all effects has no effect on this issue.

(In reply to Stefan Springer from comment #8)

Apologies for the late response. I do not use any of the mentioned effects. I've not really configured anything screen/display related - it should all be pretty much the default, apart from the theme, which is Breeze Dark. 

I just checked, the problem is still very much present. The HTML preview in Kmail is the worst problem of them all. 

Current system version:
Fedora: 40
KDE: 6.0.5
KDE Frameworks: 6.3.0
Qt Version: 6.7.1
Kernel Version: 6.9.4-200.fc40.x86_64
Wayland

I'm using a hybrid intel/nvidia GPU system, could that influence this at all? Fedora 39 and KDE 5 worked like a charm though. 

Please let me know if I need to supply any other info or test something out.

Can affected people see if this is fixed in Frameworks 6.9, to be released in three weeks? Or with git master of the qqc2-desktop-style framework, as of https://invent.kde.org/frameworks/qqc2-desktop-style/-/commit/e82957f5e6fc72e446239e2ee5139b93d3ceac85

Thanks!

🐛🧹 ⚠️ This bug has been in NEEDSINFO status with no change for at least 15 days. Please provide the requested information, then set the bug status to REPORTED. If there is no change for at least 30 days, it will be automatically closed as RESOLVED WORKSFORME.

For more information about our bug triaging procedures, please read https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging.

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

(In reply to Nate Graham from comment #11)
> Can affected people see if this is fixed in Frameworks 6.9, to be released
> in three weeks? Or with git master of the qqc2-desktop-style framework, as
> of
> https://invent.kde.org/frameworks/qqc2-desktop-style/-/commit/
> e82957f5e6fc72e446239e2ee5139b93d3ceac85
> 
> Thanks!

Apologies, I can't help anymore. I moved away from Fedora 40/KDE 6.x. as I experienced too many problems on my system (especially with Wayland vs older wacom tables and various graphical applications). Great to see people have been working on it though!

🐛🧹 ⚠️ This bug has been in NEEDSINFO status with no change for at least 15 days. Please provide the requested information, then set the bug status to REPORTED. If there is no change for at least 30 days, it will be automatically closed as RESOLVED WORKSFORME.

For more information about our bug triaging procedures, please read https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging.

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

🐛🧹 This bug has been in NEEDSINFO status with no change for at least 30 days. Closing as RESOLVED WORKSFORME.