Align dash pattern to line length performance issues

[aydin-gis]

Turning on the option for Align dash pattern to line length results in major performance issues in navigation within the layer style manager.

How to Reproduce
New user profile
create scratch layer
draw a line
in the layer styling, add a second simple line, slightly wider than the first.
make this one a dashed line. Align the dash patter to line length.
Canvas refreshes are unaffected, but navigation in the layer styling panel severely affected. Up to 5 seconds to make changes and refresh screens in the style manager.
Including Exception Bad Allocation error.

QGIS and OS versions

QGIS version
3.18.0-Zürich
QGIS code revision
bdef9fb
Compiled against Qt
5.11.2
Running against Qt
5.11.2
Compiled against GDAL/OGR
3.1.4
Running against GDAL/OGR
3.1.4
Compiled against GEOS
3.8.1-CAPI-1.13.3
Running against GEOS
3.8.1-CAPI-1.13.3
Compiled against SQLite
3.29.0
Running against SQLite
3.29.0
PostgreSQL Client Version
11.5
SpatiaLite Version
4.3.0
QWT Version
6.1.3
QScintilla2 Version
2.10.8
Compiled against PROJ
6.3.2
Running against PROJ
Rel. 6.3.2, May 1st, 2020
OS Version
Windows 10 (10.0)
Active python plugins
AnotherDXF2Shape;
pointsamplingtool;
profiletool;
quick_map_services;
db_manager;
MetaSearch;
processing

Additional context

[nyalldawson]

Can you share the style.xml definition of this symbol please?

[nyalldawson]

There's no attachment there...

[aydin-gis]

Trying again. As a zip file.
[
track.zip
](url)

[gislars]

I'd like to add my findings. Using a data defined override I've experienced the same or similar issue. The line dash pattern comes from an data defined override and if null values or values like "0;0" are present, memory and CPU usage rises until QGis crashes.

<style type="text/css"> p, li { white-space: pre-wrap; } </style>

QGIS version	3.16.11-Hannover	QGIS code revision	26cc1c7
Compiled against Qt	5.12.8	Running against Qt	5.12.8
Compiled against GDAL/OGR	3.0.4	Running against GDAL/OGR	3.0.4
Compiled against GEOS	3.8.0-CAPI-1.13.1	Running against GEOS	3.8.0-CAPI-1.13.1
Compiled against SQLite	3.31.1	Running against SQLite	3.31.1
PostgreSQL Client Version	12.8 (Ubuntu 12.8-0ubuntu0.20.04.1)	SpatiaLite Version	4.3.0a
QWT Version	6.1.4	QScintilla2 Version	2.11.2
Compiled against PROJ	6.3.1	Running against PROJ	Rel. 6.3.1, February 10th, 2020
OS Version	Linux Mint 20.2

[nyalldawson]

The original issue was fixed by #45233 -- it was caused by the symbol sizes in meters at scale.