Get the undecorated window size on GTK when saving the window config

The window decorations introduce an offset to our window size, since
they are not used when restoring the window size at startup they are
added by the window manager again.

Fixes https://gitlab.com/kicad/code/kicad/issues/4278

common/eda_base_frame.cpp

@@ -474,7 +474,7 @@ void EDA_BASE_FRAME::SaveWindowSettings( WINDOW_SETTINGS* aCfg )
     }
     else
     {
-        m_FrameSize = GetSize();
+        m_FrameSize = GetWindowSize();
         m_FramePos  = GetPosition();
     }
 
@@ -776,7 +776,7 @@ void EDA_BASE_FRAME::OnMaximize( wxMaximizeEvent& aEvent )
     // size information when we maximize the window.
     if( !IsMaximized() )
     {
-        m_NormalFrameSize = GetSize();
+        m_NormalFrameSize = GetWindowSize();
         m_NormalFramePos  = GetPosition();
         wxLogTrace( traceDisplayLocation, "Maximizing window - Saving position (%d, %d) with size (%d, %d)",
                 m_NormalFramePos.x, m_NormalFramePos.y, m_NormalFrameSize.x, m_NormalFrameSize.y );
@@ -785,3 +785,22 @@ void EDA_BASE_FRAME::OnMaximize( wxMaximizeEvent& aEvent )
     // Skip event to actually maximize the window
     aEvent.Skip();
 }
+
+
+wxSize EDA_BASE_FRAME::GetWindowSize()
+{
+    #ifdef __WXGTK__
+        // GTK includes the window decorations in the normal GetSize call,
+        // so we have to use a GTK-specific sizing call that returns the
+        // non-decorated window size.
+        int width  = 0;
+        int height = 0;
+        GTKDoGetSize( &width, &height );
+
+        wxSize winSize( width, height );
+    #else
+        wxSize winSize = GetSize();
+    #endif
+
+    return winSize;
+}

include/eda_base_frame.h

@@ -490,6 +490,16 @@ public:
      * @return true if the contents of the frame have not been saved
      */
     virtual bool IsContentModified();
+
+    /**
+     * Get the undecorated window size that can be used for restoring the window size.
+     *
+     * This is needed for GTK, since the normal wxWidgets GetSize() call will return
+     * a window size that includes the window decorations added by the window manager.
+     *
+     * @return the undecorated window size
+     */
+    wxSize GetWindowSize();
 };