[Python] PyQt6 QGraphicsView: Zoom at mouse position resets to center with customized panning

Problem Description


I'm implementing a custom PyQt6 QGraphicsView with zooming and panning functionality. When I zoom in or out using the mouse wheel, the view correctly scales but then immediately resets to center on the scene instead of maintaining the position under the mouse cursor.

Minimal Reproducible Example


import sys
from PyQt6 import QtCore, QtGui, QtWidgets
import math

class CustomGraphicsView(QtWidgets.QGraphicsView):
def __init__(self):
super().__init__()

# Setup scene
self.scene = QtWidgets.QGraphicsScene()
self.setScene(self.scene)

# Add some items to the scene
for i in range(10):
for j in range(10):
rect = QtWidgets.QGraphicsRectItem(i*100, j*100, 50, 50)
self.scene.addItem(rect)

# Center point in red
center = QtWidgets.QGraphicsEllipseItem(-10, -10, 20, 20)
center.setBrush(QtGui.QBrush(QtGui.QColor(255, 0, 0)))
self.scene.addItem(center)

# Zoom settings
self._zoom = 0
self.MAX_ZOOM = 10
self.MIN_ZOOM = -10

# Set very large scene rect
self.setSceneRect(-float("inf"), -float("inf"), float("inf"), float("inf"))

# Configure view
self.setTransformationAnchor(QtWidgets.QGraphicsView.ViewportAnchor.AnchorUnderMouse)
self.setResizeAnchor(QtWidgets.QGraphicsView.ViewportAnchor.AnchorUnderMouse)
self.setVerticalScrollBarPolicy(QtCore.Qt.ScrollBarPolicy.ScrollBarAlwaysOff)
self.setHorizontalScrollBarPolicy(QtCore.Qt.ScrollBarPolicy.ScrollBarAlwaysOff)
self.setDragMode(QtWidgets.QGraphicsView.DragMode.RubberBandDrag)
self.setMouseTracking(True)

# Panning settings
self._ctrlLeftDrag = False
self._lastDragPoint = QtCore.QPoint()

# Set initial scrollbar ranges
self.updateScrollBarRanges()

def wheelEvent(self, event: QtGui.QWheelEvent):
zoomInFactor = 1.25
zoomOutFactor = 1 / zoomInFactor

# Get the scene position before scaling
oldPos = self.mapToScene(event.position().toPoint())

# Determine zoom direction
if event.angleDelta().y() > 0:
zoomFactor = zoomInFactor
new_zoom = self._zoom + 1
else:
zoomFactor = zoomOutFactor
new_zoom = self._zoom - 1

# Check zoom limits
if new_zoom > self.MAX_ZOOM or new_zoom < self.MIN_ZOOM:
return

# Apply zoom
self.scale(zoomFactor, zoomFactor)
self._zoom = new_zoom

# Get the new position under mouse after scaling
newPos = self.mapToScene(event.position().toPoint())

# Move scene to delta to keep mouse position fixed
delta = newPos - oldPos
self.translate(delta.x(), delta.y())

# Adjust scrollbar ranges based on zoom level
self.updateScrollBarRanges()

def updateScrollBarRanges(self):
# Get current transform scale
scale = self.transform().m11()

# Base range that stays within 32-bit integer limits
MAX_RANGE = 2000000000

# Calculate new range based on scale
adjusted_range = int(min(MAX_RANGE * (1 / scale), MAX_RANGE))

# Set new ranges for both scrollbars
self.horizontalScrollBar().setRange(-adjusted_range, adjusted_range)
self.verticalScrollBar().setRange(-adjusted_range, adjusted_range)

def mousePressEvent(self, event: QtGui.QMouseEvent):
# Check for Ctrl + Left Button for panning
if (event.modifiers() == QtCore.Qt.KeyboardModifier.ControlModifier and
event.button() == QtCore.Qt.MouseButton.LeftButton):
self._ctrlLeftDrag = True
self._lastDragPoint = event.pos()
self.viewport().setCursor(QtGui.QCursor(QtCore.Qt.CursorShape.ClosedHandCursor))
event.accept()
return
# Middle mouse button for panning
elif event.button() == QtCore.Qt.MouseButton.MiddleButton:
self._ctrlLeftDrag = True
self._lastDragPoint = event.pos()
self.viewport().setCursor(QtGui.QCursor(QtCore.Qt.CursorShape.ClosedHandCursor))
event.accept()
return
super().mousePressEvent(event)

def mouseMoveEvent(self, event: QtGui.QMouseEvent):
if self._ctrlLeftDrag:
delta = event.pos() - self._lastDragPoint
self._lastDragPoint = event.pos()

# Use horizontal and vertical scrollbars for smooth scrolling
self.horizontalScrollBar().setValue(
self.horizontalScrollBar().value() - delta.x()
)
self.verticalScrollBar().setValue(
self.verticalScrollBar().value() - delta.y()
)
event.accept()
return
super().mouseMoveEvent(event)

def mouseReleaseEvent(self, event: QtGui.QMouseEvent):
if ((event.button() == QtCore.Qt.MouseButton.LeftButton or
event.button() == QtCore.Qt.MouseButton.MiddleButton) and
self._ctrlLeftDrag):
self._ctrlLeftDrag = False
self.viewport().unsetCursor()
event.accept()
return
super().mouseReleaseEvent(event)

def keyPressEvent(self, event: QtGui.QKeyEvent):
if event.key() == QtCore.Qt.Key.Key_Control:
self.viewport().setCursor(QtGui.QCursor(QtCore.Qt.CursorShape.OpenHandCursor))
super().keyPressEvent(event)

def keyReleaseEvent(self, event: QtGui.QKeyEvent):
if event.key() == QtCore.Qt.Key.Key_Control:
self.viewport().unsetCursor()
super().keyReleaseEvent(event)


if __name__ == "__main__":
app = QtWidgets.QApplication(sys.argv)
view = CustomGraphicsView()
view.show()
view.resize(800, 600)
sys.exit(app.exec())

The Issue


When running this code:

1. Use the mouse wheel to zoom in or out
2. The view initially zooms correctly at the mouse position
3. But then immediately resets/centers the view, losing the mouse position as the zoom anchor

What I've Tried


I've attempted several approaches:

• Disabling transformation anchors and manually applying translations
• Blocking scrollbar signals during range updates
• Saving view center points and restoring them
• Setting custom transformation anchors (currently using AnchorUnderMouse)

None of these solutions worked - the view still resets to the center of the scene after zooming, while correctly applying the zoom level.

Important Requirements

1. The zoom center must stay under the mouse cursor position
2. Ctrl+Left-click and Middle-click panning must continue to work
3. The scrollbar ranges need to be updated during zoom for large scenes
4. The view should not freeze when panning after zooming

Question


How can I implement a QGraphicsView zoom function that maintains the cursor position as the zoom anchor point without resetting to the center of the scene, while correctly updating scrollbar ranges and preserving my custom panning functionality?

Environment

• PyQt6 6.5.0
• Python 3.10

** I noticed the problem happens when the scrollbars do not have any scroll space **

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