diff --git a/.github/workflows/ets-from-source.yml b/.github/workflows/ets-from-source.yml
index 86e504687..dacf565da 100644
--- a/.github/workflows/ets-from-source.yml
+++ b/.github/workflows/ets-from-source.yml
@@ -13,7 +13,7 @@ jobs:
   test-with-edm:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-18.04, macos-latest, windows-latest]
         toolkit: ['wx', 'pyqt5', 'pyside2']
     runs-on: ${{ matrix.os }}
     env:
@@ -42,6 +42,7 @@ jobs:
       - name: Install Wx dependencies for Linux
         run: |
           sudo apt-get update
+          sudo apt-get install libsdl-image1.2
           sudo apt-get install libsdl2-2.0-0
         shell: bash
         if: startsWith(matrix.os, 'ubuntu') && matrix.toolkit == 'wx'
diff --git a/.github/workflows/test-with-edm.yml b/.github/workflows/test-with-edm.yml
index 3a6295b4b..14e5627dd 100644
--- a/.github/workflows/test-with-edm.yml
+++ b/.github/workflows/test-with-edm.yml
@@ -15,7 +15,7 @@ jobs:
   test-with-edm:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-18.04, macos-latest, windows-latest]
         toolkit: ['wx', 'pyqt5', 'pyside2']
     runs-on: ${{ matrix.os }}
     env:
@@ -44,6 +44,7 @@ jobs:
       - name: Install Wx dependencies for Linux
         run: |
           sudo apt-get update
+          sudo apt-get install libsdl-image1.2
           sudo apt-get install libsdl2-2.0-0
         shell: bash
         if: startsWith(matrix.os, 'ubuntu') && matrix.toolkit == 'wx'
diff --git a/etstool.py b/etstool.py
index a22c2b2b4..6f9492705 100644
--- a/etstool.py
+++ b/etstool.py
@@ -210,7 +210,7 @@ def install(edm, runtime, toolkit, environment, editable, source):
         elif sys.platform == "linux":
             # XXX this is mainly for TravisCI workers; need a generic solution
             commands.append(
-                "{edm} run -e {environment} -- pip install -f https://extras.wxpython.org/wxPython4/extras/linux/gtk3/ubuntu-16.04/ wxPython<4.1"  # noqa: E501
+                "{edm} run -e {environment} -- pip install -f https://extras.wxpython.org/wxPython4/extras/linux/gtk3/ubuntu-18.04/ wxPython<4.1"  # noqa: E501
             )
         else:
             commands.append(
diff --git a/pyface/dock/dock_window.py b/pyface/dock/dock_window.py
index 3c820a01c..ac5c8bbf9 100644
--- a/pyface/dock/dock_window.py
+++ b/pyface/dock/dock_window.py
@@ -35,6 +35,7 @@
     Str,
     List,
     Bool,
+    observe,
 )
 from traits.trait_base import traits_home
 from traitsui.api import View, HGroup, VGroup, Item, Handler, error
@@ -343,7 +344,9 @@ def _theme_default(self):
 
     # -- Trait Event Handlers ---------------------------------------------------
 
-    def _theme_changed(self, theme):
+    @observe("theme")
+    def _update_background_color_and_layout(self, event):
+        theme = event.new
         if self.control is not None:
             if theme.use_theme_color:
                 color = theme.tab.image_slice.bg_color
diff --git a/pyface/dock/dock_window_feature.py b/pyface/dock/dock_window_feature.py
index 7a16c3238..80d2fa9a2 100644
--- a/pyface/dock/dock_window_feature.py
+++ b/pyface/dock/dock_window_feature.py
@@ -24,7 +24,9 @@
 
 from weakref import ref
 
-from traits.api import HasPrivateTraits, Instance, Int, Str, Bool, Property
+from traits.api import (
+    HasPrivateTraits, Instance, Int, Str, Bool, Property, observe
+)
 from traitsui.menu import Menu, Action
 
 from pyface.timer.api import do_later
@@ -785,7 +787,8 @@ def toggle_feature(cls, event):
     #  Handles the 'image' trait being changed:
     # ---------------------------------------------------------------------------
 
-    def _image_changed(self):
+    @observe('image')
+    def _reset_bitmap(self, event):
         self._bitmap = None
 
     # -- Property Implementations ---------------------------------------------------
diff --git a/pyface/tasks/task_window.py b/pyface/tasks/task_window.py
index e15be5c2b..57ccf8eaa 100644
--- a/pyface/tasks/task_window.py
+++ b/pyface/tasks/task_window.py
@@ -416,7 +416,9 @@ def _set_title(self, title):
 
     # Trait change handlers ------------------------------------------------
 
-    def __active_state_changed(self, state):
+    @observe("_active_state")
+    def _update_traits_given_new_active_state(self, event):
+        state = event.new
         if state is None:
             self.active_task = self.central_pane = None
             self.dock_panes = []
diff --git a/pyface/tests/test_widget.py b/pyface/tests/test_widget.py
index 955929465..88ee30d1b 100644
--- a/pyface/tests/test_widget.py
+++ b/pyface/tests/test_widget.py
@@ -24,6 +24,7 @@
 
 is_qt = (toolkit_object.toolkit in {"qt4", "qt"})
 is_linux = (sys.platform == "linux")
+is_mac = (sys.platform == "darwin")
 
 
 class ConcreteWidget(Widget):
@@ -320,9 +321,9 @@ def test_enabled(self):
 
         self.assertFalse(self.widget.control.isEnabled())
 
-    @unittest.skipIf(
-        is_linux,
-        "Linux keyboard focus is False unless window is active",
+    @unittest.skipUnless(
+        is_mac,
+        "Broken on Linux and Windows",
     )
     def test_focus(self):
         with self.event_loop():
diff --git a/pyface/tree/node_manager.py b/pyface/tree/node_manager.py
index 12d2c84bd..9acc3ff7d 100644
--- a/pyface/tree/node_manager.py
+++ b/pyface/tree/node_manager.py
@@ -14,7 +14,7 @@
 import logging
 
 
-from traits.api import HasPrivateTraits, List
+from traits.api import HasPrivateTraits, List, observe
 
 
 from .node_type import NodeType
@@ -122,9 +122,10 @@ def get_key(self, node):
     # Private interface.
     # ------------------------------------------------------------------------
 
-    def _node_types_changed(self, new):
+    @observe("node_types")
+    def _update_node_manager_on_new_node_types(self, event):
         """ Called when the entire list of node types has been changed. """
-
+        new = event.new
         for node_type in new:
             node_type.node_manager = self
 
diff --git a/pyface/tree/node_tree.py b/pyface/tree/node_tree.py
index 5d48169f9..15dc92bb4 100644
--- a/pyface/tree/node_tree.py
+++ b/pyface/tree/node_tree.py
@@ -16,7 +16,7 @@
 
 
 from pyface.action.api import ActionEvent
-from traits.api import Instance, List, Property
+from traits.api import Instance, List, observe, Property
 
 
 from .node_manager import NodeManager
@@ -79,16 +79,18 @@ def _set_node_types(self, node_types):
 
     # Trait event handlers -------------------------------------------------
 
-    def _node_activated_changed(self, obj):
+    @observe("node_activated")
+    def _perform_default_action_on_activated_node(self, event):
         """ Called when a node has been activated (i.e., double-clicked). """
-
+        obj = event.new
         default_action = self.model.get_default_action(obj)
         if default_action is not None:
             self._perform_default_action(default_action, obj)
 
-    def _node_right_clicked_changed(self, obj_point):
+    @observe("node_right_clicked")
+    def _show_menu_on_right_clicked_object(self, event):
         """ Called when the right mouse button is clicked on the tree. """
-
+        obj_point = event.new
         obj, point = obj_point
         # Add the node that the right-click occurred on to the selection.
         self.select(obj)
diff --git a/pyface/ui/qt4/gui.py b/pyface/ui/qt4/gui.py
index 9749ad627..ea2a69bd3 100644
--- a/pyface/ui/qt4/gui.py
+++ b/pyface/ui/qt4/gui.py
@@ -18,7 +18,7 @@
 from pyface.qt import QtCore, QtGui
 
 
-from traits.api import Bool, HasTraits, provides, Str
+from traits.api import Bool, HasTraits, observe, provides, Str
 from pyface.util.guisupport import start_event_loop_qt4
 
 
@@ -120,9 +120,10 @@ def _state_location_default(self):
 
         return self._default_state_location()
 
-    def _busy_changed(self, new):
+    @observe("busy")
+    def _update_busy_state(self, event):
         """ The busy trait change handler. """
-
+        new = event.new
         if new:
             QtGui.QApplication.setOverrideCursor(QtCore.Qt.WaitCursor)
         else:
diff --git a/pyface/ui/qt4/heading_text.py b/pyface/ui/qt4/heading_text.py
index 227594578..ba9fc2085 100644
--- a/pyface/ui/qt4/heading_text.py
+++ b/pyface/ui/qt4/heading_text.py
@@ -15,7 +15,7 @@
 from pyface.qt import QtGui
 
 
-from traits.api import Int, provides, Str
+from traits.api import Int, observe, provides, Str
 
 
 from pyface.i_heading_text import IHeadingText, MHeadingText
@@ -71,8 +71,9 @@ def _set_text(self, text):
 
     # Trait event handlers -------------------------------------------------
 
-    def _text_changed(self, new):
+    @observe("text")
+    def _update_text(self, event):
         """ Called when the text is changed. """
-
+        new = event.new
         if self.control is not None:
             self._set_text(new)
diff --git a/pyface/ui/qt4/tasks/advanced_editor_area_pane.py b/pyface/ui/qt4/tasks/advanced_editor_area_pane.py
index e8dcf6184..b307fdd3b 100644
--- a/pyface/ui/qt4/tasks/advanced_editor_area_pane.py
+++ b/pyface/ui/qt4/tasks/advanced_editor_area_pane.py
@@ -280,7 +280,7 @@ def __init__(self, editor_area, parent=None):
                 break
 
         # Monitor focus changes so we can set the active editor.
-        QtGui.QApplication.instance().focusChanged.connect(self._focus_changed)
+        QtGui.QApplication.instance().focusChanged.connect(self._update_active_editor)
 
         # Configure the QMainWindow.
         # FIXME: Currently animation is not supported.
@@ -296,7 +296,7 @@ def __init__(self, editor_area, parent=None):
     def _remove_event_listeners(self):
         """ Disconnects focusChanged signal of the application """
         app = QtGui.QApplication.instance()
-        app.focusChanged.disconnect(self._focus_changed)
+        app.focusChanged.disconnect(self._update_active_editor)
 
     def add_editor_widget(self, editor_widget):
         """ Adds a dock widget to the editor area.
@@ -458,7 +458,7 @@ def childEvent(self, event):
                 # Use UniqueConnections since Qt recycles the tab bars.
                 child.installEventFilter(self)
                 child.currentChanged.connect(
-                    self._tab_index_changed, QtCore.Qt.UniqueConnection
+                    self._update_editor_in_focus, QtCore.Qt.UniqueConnection
                 )
                 child.setTabsClosable(True)
                 child.setUsesScrollButtons(True)
@@ -575,7 +575,7 @@ def focusInEvent(self, event):
     # Signal handlers.
     # ------------------------------------------------------------------------
 
-    def _focus_changed(self, old, new):
+    def _update_active_editor(self, old, new):
         """ Handle an application-level focus change.
         """
         if new is not None:
@@ -588,7 +588,7 @@ def _focus_changed(self, old, new):
                 if not self.editor_area.editors:
                     self.editor_area.active_editor = None
 
-    def _tab_index_changed(self, index):
+    def _update_editor_in_focus(self, index):
         """ Handle a tab selection.
         """
         widgets = self.get_dock_widgets_for_bar(self.sender())
diff --git a/pyface/undo/undo_manager.py b/pyface/undo/undo_manager.py
index 3815d077b..cb382a93d 100644
--- a/pyface/undo/undo_manager.py
+++ b/pyface/undo/undo_manager.py
@@ -32,6 +32,7 @@
     Property,
     Str,
     provides,
+    observe,
 )
 
 # Local imports.
@@ -95,9 +96,10 @@ def undo(self):
     # Private interface.
     ###########################################################################
 
-    def _active_stack_changed(self, new):
+    @observe("active_stack")
+    def _update_stack_updated(self, event):
         """ Handle a different stack becoming active. """
-
+        new = event.new
         # Pretend that the stack contents have changed.
         self.stack_updated = new
 
diff --git a/pyface/wizard/chained_wizard.py b/pyface/wizard/chained_wizard.py
index 6d0753927..788a18f9b 100644
--- a/pyface/wizard/chained_wizard.py
+++ b/pyface/wizard/chained_wizard.py
@@ -11,7 +11,7 @@
 """ A wizard model that can be chained with other wizards. """
 
 
-from traits.api import Instance
+from traits.api import Instance, observe
 
 
 from .i_wizard import IWizard
@@ -43,9 +43,10 @@ def _controller_default(self):
 
     # Static ----
 
-    def _next_wizard_changed(self, old, new):
+    @observe("next_wizard")
+    def _reset_next_controller_and_update(self, event):
         """ Handle the next wizard being changed. """
-
+        old, new = event.old, event.new
         if new is not None:
             self.controller.next_controller = new.controller
 
@@ -58,9 +59,10 @@ def _next_wizard_changed(self, old, new):
             # self._create_buttons(self.control)
             self._update()
 
-    def _controller_changed(self, old, new):
+    @observe("controller")
+    def _reset_traits_on_controller_and_update(self, event):
         """ handle the controller being changed. """
-
+        old, new = event.old, event.new
         if new is not None and self.next_wizard is not None:
             self.controller.next_controller = self.next_wizard.controller
 
diff --git a/pyface/wizard/chained_wizard_controller.py b/pyface/wizard/chained_wizard_controller.py
index cdb17a9fc..19682684c 100644
--- a/pyface/wizard/chained_wizard_controller.py
+++ b/pyface/wizard/chained_wizard_controller.py
@@ -11,7 +11,7 @@
 """ A wizard controller that can be chained with others. """
 
 
-from traits.api import Instance
+from traits.api import Instance, observe
 
 
 from .i_wizard_controller import IWizardController
@@ -168,9 +168,10 @@ def _update(self):
 
     # Static ----
 
-    def _current_page_changed(self, old, new):
+    @observe("current_paage")
+    def _reset_observers_on_current_page_and_update(self, event):
         """ Called when the current page is changed. """
-
+        old, new = event.old, event.new
         if old is not None:
             old.observe(
                 self._on_page_complete, "complete", remove=True
@@ -184,9 +185,10 @@ def _current_page_changed(self, old, new):
 
         self._update()
 
-    def _next_controller_changed(self, old, new):
+    @observe("next_controller")
+    def _reset_observers_on_next_controller_and_update(self, event):
         """ Called when the next controller is changed. """
-
+        old, new = event.old, event.new
         if old is not None:
             old.observe(
                 self._on_controller_complete, "complete", remove=True
diff --git a/pyface/wizard/wizard_controller.py b/pyface/wizard/wizard_controller.py
index b8f0afd5c..ce3d3fc6f 100644
--- a/pyface/wizard/wizard_controller.py
+++ b/pyface/wizard/wizard_controller.py
@@ -11,7 +11,9 @@
 """ A wizard controller that has a static list of pages. """
 
 
-from traits.api import Bool, HasTraits, Instance, List, Property, provides
+from traits.api import (
+    Bool, HasTraits, Instance, List, Property, provides, observe
+)
 
 
 from .i_wizard_controller import IWizardController
@@ -153,9 +155,10 @@ def _update(self):
 
     # Static ----
 
-    def _current_page_changed(self, old, new):
+    @observe("current_page")
+    def _reset_observers_on_current_page_and_update(self, event):
         """ Called when the current page is changed. """
-
+        old, new = event.old, event.new
         if old is not None:
             old.observe(
                 self._on_page_complete, "complete", remove=True