RTree: Fix iterator in single branch trees

In a single branch tree, we need to verify that all leaves do not match
the input search before returning.

Fixes #3764 | https://gitlab.com/kicad/code/kicad/issues/3764

qa/eeschema/test_sch_rtree.cpp

@@ -241,4 +241,34 @@ BOOST_AUTO_TEST_CASE( MixedElements )
     BOOST_CHECK_EQUAL( count, 1 );
 }
 
+// This tests the case where the tree has no branches but we want to iterator over a subset
+// where the first case may or may not match
+BOOST_AUTO_TEST_CASE( SingleElementTree )
+{
+    SCH_JUNCTION* junction = new SCH_JUNCTION( wxPoint( Mils2iu( 100 ), Mils2iu( 100 ) ) );
+    m_tree.insert( junction );
+
+    SCH_NO_CONNECT* nc = new SCH_NO_CONNECT( wxPoint( Mils2iu( 150 ), Mils2iu( 150 ) ) );
+    m_tree.insert( nc );
+
+    int count = 0;
+
+    for( auto item : m_tree.OfType( SCH_JUNCTION_T ) )
+    {
+        static_cast<void>( item );
+        count++;
+    }
+
+    BOOST_CHECK_EQUAL( count, 1 );
+
+    count = 0;
+    for( auto item : m_tree.OfType( SCH_NO_CONNECT_T ) )
+    {
+        static_cast<void>( item );
+        count++;
+    }
+
+    BOOST_CHECK_EQUAL( count, 1 );
+}
+
 BOOST_AUTO_TEST_SUITE_END()

thirdparty/rtree/geometry/rtree.h

@@ -404,18 +404,17 @@ public:
     {
         iterator retval( aRect );
 
-        // Only a single element in the tree
-        if( m_root->IsLeaf() )
-        {
-            if( m_root->m_count && Overlap( &aRect, &m_root->m_branch[0].m_rect ) )
-                retval.Push( m_root, 0 );
-
+        if( !m_root->m_count )
             return retval;
-        }
 
         retval.Push( m_root, 0 );
-        ++retval;
 
+        // If the first leaf matches, return the root pointer, otherwise,
+        // increment to the first match or empty if none.
+        if( m_root->IsLeaf() && Overlap( &aRect, &m_root->m_branch[0].m_rect ) )
+            return retval;
+
+        ++retval;
         return retval;
     }