Support fully-fractional HiDPI added in Qt 5.14.

QuLogic · QuLogic · commit 64be0e446c5b · 2020-11-03T00:17:31.000-05:00
In order to support fractional HiDPI without rounding, Qt 5.14 added a new policy [1], which allows devicePixelRatioF to actually return fractions, instead of rounded integers. [1] https://doc.qt.io/qt-5/qguiapplication.html#setHighDpiScaleFactorRoundingPolicy
diff --git a/lib/matplotlib/backends/backend_qt5.py b/lib/matplotlib/backends/backend_qt5.py
@@ -119,6 +119,11 @@ def _create_qApp():
                     QtCore.Qt.AA_EnableHighDpiScaling)
             except AttributeError:  # Attribute only exists for Qt>=5.6.
                 pass
+            try:
+                QtWidgets.QApplication.setHighDpiScaleFactorRoundingPolicy(
+                    QtCore.Qt.HighDpiScaleFactorRoundingPolicy.PassThrough)
+            except AttributeError:  # Added in Qt>=5.14.
+                pass
             qApp = QtWidgets.QApplication(["matplotlib"])
             qApp.lastWindowClosed.connect(qApp.quit)
             cbook._setup_new_guiapp()
