Adjust indent and spacing after DPI change if necessary

Although wxGenericTreeCtrl is not used under MSW by default, it still
doesn't hurt to adjust its metrics when DPI changes there.

Note that this shouldn't be done for the platforms using DIPs.

include/wx/generic/treectlg.h

@@ -357,6 +357,8 @@ protected:
     virtual wxSize DoGetBestSize() const override;
 
 private:
+    void OnDPIChanged(wxDPIChangedEvent& event);
+
     void OnSysColourChanged(wxSysColourChangedEvent& WXUNUSED(event))
     {
         InitVisualAttributes();

src/generic/treectlg.cpp

@@ -947,6 +947,7 @@ wxBEGIN_EVENT_TABLE(wxGenericTreeCtrl, wxTreeCtrlBase)
     EVT_KILL_FOCUS     (wxGenericTreeCtrl::OnKillFocus)
     EVT_TREE_ITEM_GETTOOLTIP(wxID_ANY, wxGenericTreeCtrl::OnGetToolTip)
     EVT_SYS_COLOUR_CHANGED(wxGenericTreeCtrl::OnSysColourChanged)
+    EVT_DPI_CHANGED(wxGenericTreeCtrl::OnDPIChanged)
 wxEND_EVENT_TABLE()
 
 // -----------------------------------------------------------------------------
@@ -4208,4 +4209,16 @@ wxSize wxGenericTreeCtrl::DoGetBestSize() const
     return size;
 }
 
+void wxGenericTreeCtrl::OnDPIChanged(wxDPIChangedEvent& event)
+{
+    // For the platforms using DPI-dependent pixels we need to adjust various
+    // metrics after the DPI change.
+#ifndef wxHAS_DPI_INDEPENDENT_PIXELS
+    m_indent = event.ScaleX(m_indent);
+    m_spacing = event.ScaleX(m_spacing);
+#endif // !wxHAS_DPI_INDEPENDENT_PIXELS
+
+    event.Skip();
+}
+
 #endif // wxUSE_TREECTRL