title: Resizing in Kivy makes blank space between widgets
body: <p>I've created a basic GUI, to test scrollview and get a platform to build on further.
  When resizing my window, there comes black space above my widgets, and there is also a lot of black space between them.</p>
  
  <p>I've tried with "anchor" and "pos" but can't get the widgets to stay in the top of the window.</p>
  
  <pre><code>class GUI(App):
      def build(self):
          root = FloatLayout(size_hint=(1, None), size=(Window.width, Window.height))
          sidepanel = BoxLayout(orientation="vertical", size_hint=(None, 1), pos_hint={"x": 0.0, "y": 0.0})
          scroll = ScrollView(pos_hint={"x": 0.2, "y": 0.0})
          layout = BoxLayout(orientation="vertical", spacing=10, size_hint_y=None)
  
          layout.bind(minimum_height=layout.setter('height'))
  
          for i in range(50):
              btn = Button(text=str(i), size_hint_y=None, height=30)
              layout.add_widget(btn)
  
  
          sidepanel.add_widget(Button(text="Import Photos", size_hint=(1,0.2)))
          sidepanel.add_widget(Button(text="Create Report", size_hint=(1,0.2)))
          sidepanel.add_widget(Button(text="Save?", size_hint=(1,0.2)))
          sidepanel.add_widget(Button(text="Button 4", size_hint=(1,0.2)))
  
  
          scroll.add_widget(layout)
  
  
          root.add_widget(sidepanel)
          root.add_widget(scroll)
  
          return root
  </code></pre>
  
  <p>I would expect the sidepanel to stay the same size, and the scrollview containing "layout" to resize with the window. And all widgets should stay in the top.</p>
  
  <p>Standard window</p>
  
  <p><img src="https://i.stack.imgur.com/XEJvX.png" alt=""></p>
  
  <p>Resized window</p>
  
  <p><img src="https://i.stack.imgur.com/uMPSi.png" alt=""></p>