QgsFieldCalculator misses project layers / shows wrong layer context

[jakimowb]

What is the bug or the crash?

The QgsFieldCalculator uses an expression context based on QgsProject.instance(). If the QgsVectorLayer to calculate field values for belongs to another QgsProject, the QgsFieldCalculator will show a wrong project context scope (see example below).

Using different QgsProject instances eases the handling of different layer sets, e.g. to focus on layers which are relevant for a plugin application, instead of showing all layers existing in a QgsProject.instance().
Classes like QgsMapLayerCombox or QgsVectorLayerTools implement ::setProject(QgsProject * project) which allows to benefit from specialized QgsProject instances.

This behaviour is caused by:

QgsFieldCalculator::QgsFieldCalculator( QgsVectorLayer *vl, QWidget *parent )
...
    QgsExpressionContext expContext( QgsExpressionContextUtils::globalProjectLayerScopes( mVectorLayer ) );
...

and:

QList<QgsExpressionContextScope *> QgsExpressionContextUtils::globalProjectLayerScopes( const QgsMapLayer *layer )
{
  QList<QgsExpressionContextScope *> scopes;
  scopes << globalScope();

  QgsProject *project = QgsProject::instance();  // TODO: use project associated with layer skip-keyword-check
  if ( project )
    scopes << projectScope( project );

  if ( layer )
    scopes << layerScope( layer );
  return scopes;
}

Not being a C++ developer, I recommend to replace the //TODO line
with:

QgsProject *project = layer && layer->project() ? layer->project() : QgsProject::instance();

Steps to reproduce the issue

In the following example I may want to use layers from QgsProject pB, but the QgsFieldCalculator lists only map layer from QgsProject.instance().

from qgis.core import QgsProject, QgsVectorLayer
from qgis.gui import QgsFieldCalculator
from qgis.testing import start_app

start_app()

pA = QgsProject.instance()
pB = QgsProject()

lyrA = QgsVectorLayer('Point?crs=epsg:4326&field=foo:string(20)', 'A', 'memory')
lyrB1 = QgsVectorLayer('Point?crs=epsg:4326&field=bar:string(20)', 'B1', 'memory')
lyrB2 = QgsVectorLayer('Point?crs=epsg:4326&field=bar:string(20)', 'B2', 'memory')
pA.addMapLayer(lyrA)
pB.addMapLayers([lyrB1, lyrB2])

assert pA != pB
assert lyrA.project() == pA
assert lyrB1.project() == pB
assert lyrB2.project() == pB

calc = QgsFieldCalculator(lyrB1)
calc.exec_()

Versions

QGIS version	3.44.7-Solothurn
QGIS code branch	Release 3.44
Libraries
Qt version	5.15.15
Python version	3.13.11
GDAL version	3.12.0 (Compiled) 3.12.1 (Running) — Chicoutimi
PROJ version	9.7.1
EPSG Registry database version	v12.029 (2025-10-03)
GEOS version	3.14.1-CAPI-1.20.5
SQLite version	3.51.2
PDAL version	2.9.2 (Compiled) 2.9.3 (Running)
PostgreSQL client version	18.1
SpatiaLite version	5.1.0
QWT version	6.2.0
QScintilla2 version	2.14.1
OS version	Fedora Linux 43 (Workstation Edition)
Active Python plugins
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