Bug 513424

Summary:	High plasmashell CPU usage after turning off screen (only on laptops with external monitor)
Product:	[Plasma] plasmashell	Reporter:	ch4xer <ch4xer>
Component:	generic-performance	Assignee:	Plasma Bugs List <plasma-bugs-null>
Status:	REPORTED ---
Severity:	major	CC:	ch4xer, natalie_clarius, nate
Priority:	NOR	Keywords:	multiscreen
Version First Reported In:	6.5.4
Target Milestone:	1.0
Platform:	Arch Linux
OS:	Linux
Latest Commit:		Version Fixed/Implemented In:
Sentry Crash Report:

Description ch4xer 2025-12-16 06:07:42 UTC

SUMMARY
When a laptop connected to an external monitor (both screens in use) has "Turn off screen when locked" enabled, abnormal high CPU usage (>60%) of plasmashell is observed via `nvtop` over SSH after locking the screen and turning off the display. Upon waking the screen and returning to the desktop, the desktop background and panels disappear temporarily while application windows remain visible. After a few seconds, the desktop background and panels recover, and plasmashell CPU usage drops to low levels (<1%).

This issue occurs only on a laptop connected to an external monitor. The problem disappears when the external monitor is disconnected.

Desktop computers connected to multiple monitors do not exhibit this issue.

STEPS TO REPRODUCE

1. Log into a Plasma Wayland session.
2. Connect the laptop to an external monitor and ensure both displays are active.
3. In Power Management settings, set "When locked: Immediately" under "Turn off screen".
4. Lock the screen and let the displays turn off.
5. Wait for a while.
6. SSH into the computer remotely and run `top` or `nvtop` to observe plasmashell CPU usage.

OBSERVED RESULT

1. While the screen is off, plasmashell CPU usage exceeds 60%. After waking the screen and logging into the desktop, plasmashell CPU usage drops below 1%.
2. When waking the screen and returning to the desktop, application windows appear immediately, but the desktop background and panels take several seconds to appear.

EXPECTED RESULT

1. The plasmashell process should maintain low CPU usage even when the screen is turned off.
2. The desktop (background and panels) should appear immediately upon waking the screen and logging into the desktop.

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-zen1-1-zen (64-bit)
Graphics Platform: Wayland
Processors: 20 × AMD Ryzen AI 9 365 w/ Radeon 880M
Memory: 32 GiB of RAM (27.2 GiB usable)
Graphics Processor: AMD Radeon 890M Graphics
Manufacturer: LENOVO
System Version: ThinkBook 14 G7+ ASP