Line symbol with offset is incomplete if start and end coordinates are equal

[SGroe]

What is the bug or the crash?

The first and last line segments are missing if offset is greater than 0.

(The line is complete if offset is less than 0.)

(Start-end node is indicated with green arrow, original line geometry is black, offset line is red)

Steps to reproduce the issue

1. Create or find line feature where start and end node coordinates are equal.
2. Set line offset to a value greater than 0
3. See missing line segments

Versions

QGIS-Version
3.38.3-Grenoble
QGIS-Codeversion
37f9e6e
Qt-Version
5.15.13
Python-Version
3.12.6
GDAL-Version
3.9.2
PROJ-Version
9.4.0
EPSG-Registraturdatenbankversion
v11.004 (2024-02-24)
GEOS-Version
3.12.2-CAPI-1.18.2
SQLite-Version
3.45.1
PDAL-Version
2.6.3
PostgreSQL-Client-Version
16.2
SpatiaLite-Version
5.1.0
QWT-Version
6.2.0
QScintilla2-Version
2.14.1
BS-Version
Windows 11 Version 2009

Aktive Python-Erweiterungen
db_manager
0.1.20
grassprovider
2.12.99
MetaSearch
0.3.6
processing
2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[nyalldawson]

This is an upstream (geos) issue, see libgeos/geos#1037

[agiudiceandrea]

See also #51583.

[ThiesVanLoon]

I circumvented this problem within de model designer using the following steps;

1. Multipart to singleparts (if nessesary)
2. Field calculator; is_closed($geometry) *1 which points out the line features where this bug may occur
3. Offset the lines as usual
4. Geometry by expression; If the source line was closed, the offset line should be as well. The following expression will repair the forgotten line segment by re-closing the line.
   if( "is_closed", close_line( @geometry), @geometry)