Return empty rectangle from GetItemRect() if item is not visible

This was already the case if the item was not visible because its parent
was not expanded, but now make it also true for the items which are not
visible due to the current scrollbar position.

Add unit tests checking for this and also verifying that GetItemRect()
returns the coordinates in the physical window coordinates.

tests/controls/dataviewctrltest.cpp

@@ -208,6 +208,9 @@ TEST_CASE_METHOD(SingleSelectDataViewCtrlTestCase,
     const wxRect rect1 = m_dvc->GetItemRect(m_child1);
     const wxRect rect2 = m_dvc->GetItemRect(m_child2);
 
+    CHECK( rect1 != wxRect() );
+    CHECK( rect2 != wxRect() );
+
     CHECK( rect1.x == rect2.x );
     CHECK( rect1.width == rect2.width );
     CHECK( rect1.height == rect2.height );
@@ -215,6 +218,30 @@ TEST_CASE_METHOD(SingleSelectDataViewCtrlTestCase,
 
     const wxRect rectNotShown = m_dvc->GetItemRect(m_grandchild);
     CHECK( rectNotShown == wxRect() );
+
+    // Append enough items to make the window scrollable.
+    for ( int i = 3; i < 100; ++i )
+        m_dvc->AppendItem(m_root, wxString::Format("child%d", i));
+
+    const wxDataViewItem last = m_dvc->AppendItem(m_root, "last");
+
+    // This should scroll the window to bring this item into view.
+    m_dvc->EnsureVisible(last);
+
+    // Check that this was indeed the case.
+    const wxDataViewItem top = m_dvc->GetTopItem();
+    CHECK( top != m_root );
+
+    // Verify that the coordinates are returned in physical coordinates of the
+    // window and not the logical coordinates affected by scrolling.
+    const wxRect rectScrolled = m_dvc->GetItemRect(top);
+    CHECK( rectScrolled.GetBottom() > 0 );
+    CHECK( rectScrolled.GetTop() <= m_dvc->GetClientSize().y );
+
+    // Also check that the root item is not currently visible (because it's
+    // scrolled off).
+    const wxRect rectRoot = m_dvc->GetItemRect(m_root);
+    CHECK( rectRoot == wxRect() );
 #endif
 }