remove the separator when it is at the bottom (unles the user maid M-v)

src/tabs.py

@@ -605,6 +605,8 @@ class MucTab(ChatTab):
 
     def on_gain_focus(self):
         self._room.set_color_state(theme.COLOR_TAB_CURRENT)
+        if self.text_win.built_lines[-1] is None:
+            self.text_win.remove_line_separator()
         curses.curs_set(1)
 
     def on_scroll_up(self):

src/windows.py

@@ -536,10 +536,10 @@ class TextWin(Win):
         """
         if self.height <= 0:
             return
-        if self.pos != 0:
-            lines = self.built_lines[-self.height-self.pos:-self.pos]
-        else:
+        if self.pos == 0:
             lines = self.built_lines[-self.height:]
+        else:
+            lines = self.built_lines[-self.height-self.pos:-self.pos]
         self._win.move(0, 0)
         with g_lock:
             self._win.erase()