Fix generation of extraneous wxEVT_SLIDER events in wxMSW

Don't send the event when it's redundant, i.e. doesn't really notify
about the change in the slider value.

Also add a test case for wxEVT_SLIDER and show these events in the
widgets sample.

Closes https://github.com/wxWidgets/wxWidgets/pull/2080

Closes #18929.

samples/widgets/slider.cpp

@@ -120,6 +120,7 @@ protected:
     void OnCheckOrRadioBox(wxCommandEvent& event);
 
     void OnSlider(wxScrollEvent& event);
+    void OnSlider(wxCommandEvent& event);
 
     void OnUpdateUIValueButton(wxUpdateUIEvent& event);
     void OnUpdateUIMinMaxButton(wxUpdateUIEvent& event);
@@ -230,6 +231,7 @@ wxBEGIN_EVENT_TABLE(SliderWidgetsPage, WidgetsPage)
     EVT_UPDATE_UI(SliderPage_CurValueText, SliderWidgetsPage::OnUpdateUICurValueText)
 
     EVT_COMMAND_SCROLL(SliderPage_Slider, SliderWidgetsPage::OnSlider)
+    EVT_SLIDER(SliderPage_Slider, SliderWidgetsPage::OnSlider)
 
     EVT_CHECKBOX(wxID_ANY, SliderWidgetsPage::OnCheckOrRadioBox)
     EVT_RADIOBOX(wxID_ANY, SliderWidgetsPage::OnCheckOrRadioBox)
@@ -845,4 +847,12 @@ void SliderWidgetsPage::OnSlider(wxScrollEvent& event)
                  event.GetInt());
 }
 
+void SliderWidgetsPage::OnSlider(wxCommandEvent& event)
+{
+    static int s_numSliderEvents = 0;
+
+    wxLogMessage("Slider event #%d: wxEVT_SLIDER (value = %d)",
+                 s_numSliderEvents++, event.GetInt());
+}
+
 #endif // wxUSE_SLIDER

src/msw/slider.cpp

@@ -313,15 +313,28 @@ bool wxSlider::MSWOnScroll(int WXUNUSED(orientation),
     SetValue(newPos);
 
     wxScrollEvent event(scrollEvent, m_windowId);
+    bool          processed = false;
+
     event.SetPosition(newPos);
     event.SetEventObject( this );
-    HandleWindowEvent(event);
+    processed = HandleWindowEvent(event);
 
-    wxCommandEvent cevent( wxEVT_SLIDER, GetId() );
+    // Do not generate wxEVT_SLIDER when the native scroll message
-    cevent.SetInt( newPos );
+    // parameter is SB_ENDSCROLL, which always follows only after
-    cevent.SetEventObject( this );
+    // another scroll message which already changed the slider value.
+    // Therefore, sending wxEVT_SLIDER after SB_ENDSCROLL
+    // would result in two wxEVT_SLIDER events with the same value.
+    if ( wParam != SB_ENDSCROLL )
+    {
+        wxCommandEvent cevent( wxEVT_SLIDER, GetId() );
 
-    return HandleWindowEvent( cevent );
+        cevent.SetInt( newPos );
+        cevent.SetEventObject( this );
+
+        processed = HandleWindowEvent( cevent );
+    }
+
+    return processed;
 }
 
 void wxSlider::Command (wxCommandEvent & event)

tests/controls/slidertest.cpp

@@ -35,6 +35,7 @@ private:
 #ifndef __WXOSX__
         WXUISIM_TEST( PageUpDown );
         WXUISIM_TEST( LineUpDown );
+        WXUISIM_TEST( EvtSlider );
         WXUISIM_TEST( LinePageSize );
 #endif
         CPPUNIT_TEST( Value );
@@ -47,6 +48,7 @@ private:
 
     void PageUpDown();
     void LineUpDown();
+    void EvtSlider();
     void LinePageSize();
     void Value();
     void Range();
@@ -125,6 +127,24 @@ void SliderTestCase::LineUpDown()
 #endif
 }
 
+void SliderTestCase::EvtSlider()
+{
+#if wxUSE_UIACTIONSIMULATOR
+    EventCounter slider(m_slider, wxEVT_SLIDER);
+
+    wxUIActionSimulator sim;
+    wxYield();
+    m_slider->SetFocus();
+
+    sim.Char(WXK_UP);
+    sim.Char(WXK_DOWN);
+
+    wxYield();
+
+    CPPUNIT_ASSERT_EQUAL(2, slider.GetCount());
+#endif
+}
+
 void SliderTestCase::LinePageSize()
 {
 #if wxUSE_UIACTIONSIMULATOR