https://bugs.kde.org/show_bug.cgi?id=450394
Bug ID: 450394
Summary: BambooTracker slows down on Breeze, due to leaking
QProxyStyle and Breeze installing global event
handlers
Product: Breeze
Version: 5.24.1
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: QStyle
Assignee: plasma-b...@kde.org
Reporter: nyanpas...@tuta.io
CC: noaha...@gmail.com
Target Milestone: ---
SUMMARY
When I run BambooTracker and open and close many different instrument dialogs,
the application slows down when rendering with Breeze theme.
STEPS TO REPRODUCE
1. Install/build and run https://github.com/BambooTracker/BambooTracker
(self-built or from a distro package).
2. Download and extract
https://github.com/BambooTracker/BambooTracker/files/7169359/32.64.128.instruments.zip,
and open "128 instruments.btm" in BambooTracker.
3. In the instruments pane in the top of the window, double-click each
instrument and then close the dialog (eg. pressing Esc).
The application leaks QProxyStyle (yes, it should be fixed, I reported at
https://github.com/BambooTracker/BambooTracker/issues/418). Each FM instrument
dialog creates dozens of `LabeledVerticalSlider`, each with their own
`SliderStyle : QProxyStyle` instance intended to set
`styleHint(QStyle::SH_Slider_AbsoluteSetButtons) = Qt::LeftButton`. When you
open dozens of instrument dialogs, the app becomes increasingly unresponsive as
`QCoreApplication` loops through around a thousand event filters per event.
(Due to another bug, opening and closing an instrument dialog doesn't delete
the dialog object, so you only need to open and close one dialog at a time to
get the slowdown.)
I haven't fixed the bugs in BambooTracker itself, and neither have the app's
maintainers. I suspect a lazy-init `static SliderStyle * = nullptr` might work
better than the ideas I listed in the original bug report.
OBSERVED RESULT
The application should start slowing down after around 10 instruments. By the
time you've opened 30-40 instrument dialogs, there will be a noticeable delay
(0.5-1 second) opening/closing instrument or other dialogs. The slowdown
increases as you open more instruments.
EXPECTED RESULT
Each QProxyStyle created doesn't cause a global slowdown. I don't notice any
slowdown when setting KDE to use the Fusion application style instead of
Breeze.
SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.24.1
KDE Frameworks Version: 5.91.0
Qt Version: 5.15.2
Kernel Version: 5.16.9-zen1-1-zen (64-bit)
Graphics Platform: X11
Processors: 12 × AMD Ryzen 5 5600X 6-Core Processor
Memory: 15.6 GiB of RAM
Graphics Processor: NVIDIA GeForce GT 730/PCIe/SSE2
ADDITIONAL INFORMATION
Initially reported as 442465, which was closed with the message "If you have an
example of actual apps in the wild using proxy styles and having lag due to
Breeze, please open a new bug about this."
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