Listen to screen DPI changes (#864)

Co-authored-by: Corran Webster <cwebster@enthought.com>

enable/qt4/base_window.py

@@ -31,7 +31,7 @@
 # Enthought library imports.
 from enable.abstract_window import AbstractWindow
 from enable.events import KeyEvent, MouseEvent, DragEvent
-from traits.api import Instance
+from traits.api import Instance, Property
 
 # Local imports.
 from .constants import (
@@ -351,6 +351,9 @@ class _Window(AbstractWindow):
 
     control = Instance(QtGui.QWidget)
 
+    # Use a Property since this value can change dynamically
+    base_pixel_scale = Property(observe="control")
+
     def __init__(self, parent, wid=-1, pos=None, size=None, **traits):
         AbstractWindow.__init__(self, **traits)
 
@@ -360,15 +363,21 @@ def __init__(self, parent, wid=-1, pos=None, size=None, **traits):
             parent = parent.parentWidget()
         self.control = self._create_control(parent, self)
 
-        if self.high_resolution and hasattr(self.control, "devicePixelRatio"):
-            self.base_pixel_scale = self.control.devicePixelRatio()
-
         if pos is not None:
             self.control.move(*pos)
 
         if size is not None:
             self.control.resize(*size)
 
+    def _get_base_pixel_scale(self):
+        if self.control is None:
+            return 1.0
+
+        if self.high_resolution and hasattr(self.control, "devicePixelRatio"):
+            return self.control.devicePixelRatio()
+
+        return 1.0
+
     # ------------------------------------------------------------------------
     # Implementations of abstract methods in AbstractWindow
     # ------------------------------------------------------------------------

enable/tests/test_component_editor.py

@@ -10,13 +10,16 @@
 """ Test the interaction between traitsui and enable's ComponentEditor.
 """
 import unittest
+from unittest import mock
 
 from traits.api import Any, HasTraits
 from traitsui.api import Item, View
 
 from enable.component import Component
 from enable.component_editor import ComponentEditor
-from enable.tests._testing import get_dialog_size, skip_if_null
+from enable.tests._testing import (
+    get_dialog_size, skip_if_null, skip_if_not_qt, skip_if_not_wx
+)
 
 ITEM_WIDTH, ITEM_HEIGHT = 700, 200
 
@@ -74,8 +77,8 @@ def test_initial_component_with_item_size(self):
         self.assertGreater(size[0], ITEM_WIDTH - 1)
         self.assertGreater(size[1], ITEM_HEIGHT - 1)
 
-    @skip_if_null
-    def test_component_hidpi_size_stability(self):
+    @skip_if_not_wx
+    def test_component_hidpi_size_stability_wx(self):
         # Issue #634: HiDPI doubles size of components when a component is
         # replaced
         dialog = _ComponentDialogWithSize(thing=Component())
@@ -89,3 +92,20 @@ def test_component_hidpi_size_stability(self):
         new_bounds = dialog.thing.bounds
 
         self.assertListEqual(initial_bounds, new_bounds)
+
+    @skip_if_not_qt
+    def test_component_hidpi_size_stability_qt(self):
+        # Issue #634: HiDPI doubles size of components when a component is
+        # replaced
+        dialog = _ComponentDialogWithSize(thing=Component())
+        dialog.edit_traits()
+
+        initial_bounds = dialog.thing.bounds
+        # Force the window into HiDPI mode
+        with mock.patch.object(
+            dialog.thing.window.control, "devicePixelRatio", return_value=2
+        ) as mock_method:
+            dialog.thing = Component()
+            new_bounds = dialog.thing.bounds
+
+            self.assertListEqual(initial_bounds, new_bounds)