Bug 513408

Summary:	Missing horizontal separator line under tab bars at certain display scaling values
Product:	[Plasma] Breeze	Reporter:	Mathis Paquet <mathispaquet362>
Component:	general	Assignee:	Plasma Bugs List <plasma-bugs-null>
Status:	CONFIRMED ---
Severity:	normal	CC:	alexandre, mathispaquet362, nate, uhhadd
Priority:	NOR
Version First Reported In:	6.5.4
Target Milestone:	---
Platform:	Arch Linux
OS:	Linux
Latest Commit:		Version Fixed/Implemented In:
Sentry Crash Report:
Attachments:	Mockup showing no line (current) vs. line Close up of the mockup Line visible at 200% scaling

Description Mathis Paquet 2025-12-15 16:25:09 UTC

Created attachment 187671 [details]
Mockup showing no line (current) vs. line

SUMMARY
Tab bars are supposed to have a separator line spanning the whole width of the tab bar. It seems like it used to be the case in applications such as Dolphin, but there has been a regression or a bug causing it not to appear anymore. Some apps have most of the line (like Kate) or small parts of it only (like Konsole, under the command palette button). 

STEPS TO REPRODUCE
1. Open Dolphin, Konsole, Okular or Kate.
2. Open a second tab.

OBSERVED RESULT
The horizontal separator line is absent or it does not span the whole width of the tab container.

EXPECTED RESULT
The horizontal line should consistently span the width of the tab bar container across all applications using tabs.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.5.4
KDE Frameworks Version: 6.21.0
Qt Version: 6.10.1
Kernel Version: 6.17.9-arch1-1 (64-bit)
Graphics Platform: Wayland
Processors: 32 × 13th Gen Intel® Core™ i9-13900HX
Memory: 32 GiB of RAM (31.1 GiB usable)
Graphics Processor 1: Mesa Intel® Graphics
Graphics Processor 2: NVIDIA GeForce RTX 4090 Laptop GPU/PCIe/SSE2

ADDITIONAL INFORMATION
Some users are reporting that they do see the line, notably around Plasma version 6.3.5
- https://www.reddit.com/r/kde/comments/1pn9cu1/comment/nu65jl9/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button
- https://bugs.kde.org/show_bug.cgi?id=505176
- https://www.reddit.com/r/kde/comments/1pn9cu1/comment/nu6daua/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button

Comment 1 Mathis Paquet 2025-12-15 16:26:18 UTC

Created attachment 187672 [details]
Close up of the mockup

Comment 2 Mathis Paquet 2025-12-15 16:41:51 UTC

I discovered that this is caused by scaling. For instance, I use my 16:10 1440p monitor at 125% scaling. If I set it to 200% scaling, I do see the lines.

Comment 3 Mathis Paquet 2025-12-15 16:42:35 UTC

Created attachment 187677 [details]
Line visible at 200% scaling

Comment 4 Alexandre Hen 2025-12-16 13:38:38 UTC

Hi !

I can confirm, same problem here on :

Operating System: Fedora Linux 43
KDE Plasma Version: 6.5.4
KDE Frameworks Version: 6.21.0
Qt Version: 6.10.1
Kernel Version: 6.17.11-300.fc43.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 PRO 4750U with Radeon Graphics
Memory: 32 Gio of RAM (29.1 Gio usable)
Graphics Processor: AMD Radeon Graphics
Manufacturer: LENOVO
Product Name: 20Y1CTO1WW
System Version: ThinkPad P14s Gen 1

Switching to 100% solve the problem, but is not usable on my screen.

Thank you for your report,

Alexandre