ensure scrollable lists are centered in the visible viewport

docs/changelog.txt

@@ -38,6 +38,7 @@ changelog.txt uses a syntax similar to RST, with a few special sequences:
 ## Fixes
 - ``Units::isFortControlled``: Account for agitated wildlife
 - Fix right click sometimes closing both a DFHack window and a vanilla panel
+- Fixed issue with scrollable lists having some data off-screen if they were scrolled before being made visible
 
 ## Misc Improvements
 - `automelt`: is now more resistent to savegame corruption

library/lua/gui/widgets.lua

@@ -1634,6 +1634,7 @@ end
 
 function List:postComputeFrame(body)
     self.page_size = math.max(1, math.floor(body.height / self.row_height))
+    self.page_top = math.max(1, math.min(#self.choices - self.page_size + 1))
     update_list_scrollbar(self)
 end