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qt: Use better devicePixelRatio event to refresh scaling #30345

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Merged
merged 1 commit into from
Jul 24, 2025
+25 −9
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qt: Use better devicePixelRatio event to refresh scaling #30345

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13 changes: 11 additions & 2 deletions lib/matplotlib/backends/backend_qt.py
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Original file line number Diff line number Diff line change
Expand Up @@ -262,12 +262,21 @@ def _update_screen(self, screen):
screen.physicalDotsPerInchChanged.connect(self._update_pixel_ratio)
screen.logicalDotsPerInchChanged.connect(self._update_pixel_ratio)

def eventFilter(self, source, event):
if event.type() == QtCore.QEvent.Type.DevicePixelRatioChange:
self._update_pixel_ratio()
return super().eventFilter(source, event)

def showEvent(self, event):
# Set up correct pixel ratio, and connect to any signal changes for it,
# once the window is shown (and thus has these attributes).
window = self.window().windowHandle()
window.screenChanged.connect(self._update_screen)
self._update_screen(window.screen())
current_version = tuple(int(x) for x in QtCore.qVersion().split('.', 2)[:2])
if current_version >= (6, 6):
window.installEventFilter(self)
else:
window.screenChanged.connect(self._update_screen)
self._update_screen(window.screen())
Comment on lines +275 to +279
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I suspect that both cases are a bit sloppy in that they connect/install on every showEvent. We typically have one, but there may be cases where the window lives longer and is hidden/shown multiple times. At least for connect() multiple calls result in multiple connections, i.e. firing multiple times.

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atleast for the event filter version

If filterObj has already been installed for this object, this function moves it so it acts as if it was installed last.

so it is OK to install it multiple times.

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I'm not sure if I could find a better event just yet, but I'll try to take a look again later.


def set_cursor(self, cursor):
# docstring inherited
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21 changes: 14 additions & 7 deletions lib/matplotlib/tests/test_backend_qt.py
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Original file line number Diff line number Diff line change
Expand Up @@ -137,7 +137,7 @@ def on_key_press(event):


@pytest.mark.backend('QtAgg', skip_on_importerror=True)
def test_device_pixel_ratio_change():
def test_device_pixel_ratio_change(qt_core):
"""
Make sure that if the pixel ratio changes, the figure dpi changes but the
widget remains the same logical size.
Expand All @@ -154,11 +154,19 @@ def test_device_pixel_ratio_change():
def set_device_pixel_ratio(ratio):
p.return_value = ratio

# The value here doesn't matter, as we can't mock the C++ QScreen
# object, but can override the functional wrapper around it.
# Emitting this event is simply to trigger the DPI change handler
# in Matplotlib in the same manner that it would occur normally.
screen.logicalDotsPerInchChanged.emit(96)
window = qt_canvas.window().windowHandle()
current_version = tuple(
int(x) for x in qt_core.qVersion().split('.', 2)[:2])
if current_version >= (6, 6):
qt_core.QCoreApplication.sendEvent(
window,
qt_core.QEvent(qt_core.QEvent.Type.DevicePixelRatioChange))
else:
# The value here doesn't matter, as we can't mock the C++ QScreen
# object, but can override the functional wrapper around it.
# Emitting this event is simply to trigger the DPI change handler
# in Matplotlib in the same manner that it would occur normally.
window.screen().logicalDotsPerInchChanged.emit(96)

qt_canvas.draw()
qt_canvas.flush_events()
Expand All @@ -168,7 +176,6 @@ def set_device_pixel_ratio(ratio):

qt_canvas.manager.show()
size = qt_canvas.size()
screen = qt_canvas.window().windowHandle().screen()
set_device_pixel_ratio(3)

# The DPI and the renderer width/height change
Expand Down
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