Try using the Objective-C 'alloc + init' idiom for lua screen objects.

library/lua/dfhack.lua

@@ -104,11 +104,21 @@ end
 
 -- Trivial classes
 
+function rawset_default(target,source)
+    for k,v in pairs(source) do
+        if rawget(target,k) == nil then
+            rawset(target,k,v)
+        end
+    end
+end
+
 function defclass(class,parent)
     class = class or {}
-    rawset(class, '__index', rawget(class, '__index') or class)
+    rawset_default(class, { __index = class })
     if parent then
         setmetatable(class, parent)
+    else
+        rawset_default(class, { init_fields = rawset_default })
     end
     return class
 end
@@ -153,14 +163,6 @@ function xyz2pos(x,y,z)
     end
 end
 
-function rawset_default(target,source)
-    for k,v in pairs(source) do
-        if rawget(target,k) == nil then
-            rawset(target,k,v)
-        end
-    end
-end
-
 function safe_index(obj,idx,...)
     if obj == nil or idx == nil then
         return nil
@@ -202,12 +204,6 @@ function dfhack.buildings.getSize(bld)
     return bld.x2+1-x, bld.y2+1-y, bld.centerx-x, bld.centery-y
 end
 
-dfhack.screen.__index = dfhack.screen
-
-function dfhack.screen:__tostring()
-    return "<lua viewscreen: "..tostring(self._native)..">"
-end
-
 -- Interactive
 
 local print_banner = true

library/lua/gui.lua

@@ -4,6 +4,8 @@ local _ENV = mkmodule('gui')
 
 local dscreen = dfhack.screen
 
+USE_GRAPHICS = dscreen.inGraphicsMode()
+
 CLEAR_PEN = {ch=32,fg=0,bg=0}
 
 function simulateInput(screen,...)
@@ -52,6 +54,9 @@ function inset(rect,dx1,dy1,dx2,dy2)
         rect.x2-(dx2 or dx1), rect.y2-(dy2 or dy1)
     )
 end
+function is_in_rect(rect,x,y)
+    return x and y and x >= rect.x1 and x <= rect.x2 and y >= rect.y1 and y <= rect.y2
+end
 
 local function to_pen(default, pen, bg, bold)
     if pen == nil then
@@ -201,10 +206,17 @@ end
 -- Base screen object --
 ------------------------
 
-Screen = defclass(Screen, dfhack.screen)
+Screen = defclass(Screen)
 
 Screen.text_input_mode = false
 
+function Screen:init()
+    self:updateLayout()
+    return self
+end
+
+Screen.isDismissed = dscreen.isDismissed
+
 function Screen:isShown()
     return self._native ~= nil
 end
@@ -213,6 +225,18 @@ function Screen:isActive()
     return self:isShown() and not self:isDismissed()
 end
 
+function Screen:invalidate()
+    dscreen.invalidate()
+end
+
+function Screen:getWindowSize()
+    return dscreen.getWindowSize()
+end
+
+function Screen:getMousePos()
+    return dscreen.getMousePos()
+end
+
 function Screen:renderParent()
     if self._native and self._native.parent then
         self._native.parent:render()
@@ -241,6 +265,7 @@ function Screen:onAboutToShow()
 end
 
 function Screen:onShow()
+    self:updateLayout()
 end
 
 function Screen:dismiss()
@@ -252,6 +277,13 @@ end
 function Screen:onDismiss()
 end
 
+function Screen:onResize(w,h)
+    self:updateLayout()
+end
+
+function Screen:updateLayout()
+end
+
 ------------------------
 -- Framed screen object --
 ------------------------
@@ -318,7 +350,8 @@ local function hint_coord(gap,hint)
     end
 end
 
-function FramedScreen:updateFrameSize(sw,sh)
+function FramedScreen:updateFrameSize()
+    local sw, sh = dscreen.getWindowSize()
     local iw, ih = sw-2, sh-2
     local width = math.min(self.frame_width or iw, iw)
     local height = math.min(self.frame_height or ih, ih)
@@ -328,8 +361,21 @@ function FramedScreen:updateFrameSize(sw,sh)
     self.frame_opaque = (gw == 0 and gh == 0)
 end
 
-function FramedScreen:onResize(w,h)
+function FramedScreen:updateLayout()
-    self:updateFrameSize(w,h)
+    self:updateFrameSize()
+end
+
+function FramedScreen:getWindowSize()
+    local rect = self.frame_rect
+    return rect.width, rect.height
+end
+
+function FramedScreen:getMousePos()
+    local rect = self.frame_rect
+    local x,y = dscreen.getMousePos()
+    if is_in_rect(rect,x,y) then
+        return x-rect.x1, y-rect.y1
+    end
 end
 
 function FramedScreen:onRender()
@@ -348,4 +394,7 @@ function FramedScreen:onRender()
     self:onRenderBody(Painter.new(rect))
 end
 
+function FramedScreen:onRenderBody(dc)
+end
+
 return _ENV

library/lua/gui/dwarfmode.lua

@@ -179,14 +179,6 @@ function DwarfOverlay:updateLayout()
     self.df_layout = getPanelLayout()
 end
 
-function DwarfOverlay:onShown()
-    self:updateLayout()
-end
-
-function DwarfOverlay:onResize(w,h)
-    self:updateLayout()
-end
-
 function DwarfOverlay:getViewport(old_vp)
     if old_vp then
         return old_vp:resize(self.df_layout)
@@ -238,6 +230,14 @@ end
 
 MenuOverlay = defclass(MenuOverlay, DwarfOverlay)
 
+function MenuOverlay:updateLayout()
+    DwarfOverlay.updateLayout(self)
+    self.frame_rect = self.df_layout.menu
+end
+
+MenuOverlay.getWindowSize = gui.FramedScreen.getWindowSize
+MenuOverlay.getMousePos = gui.FramedScreen.getMousePos
+
 function MenuOverlay:onAboutToShow(below)
     DwarfOverlay.onAboutToShow(self,below)
 
@@ -249,7 +249,6 @@ end
 
 function MenuOverlay:onRender()
     self:renderParent()
-    self:updateLayout()
 
     local menu = self.df_layout.menu
     if menu then

scripts/gui/hello-world.lua

@@ -17,6 +17,6 @@ local screen = mkinstance(gui.FramedScreen, {
             self:dismiss()
         end
     end
-})
+}):init()
 
 screen:show()

scripts/gui/mechanisms.lua

@@ -52,15 +52,16 @@ MechanismList = defclass(MechanismList, guidm.MenuOverlay)
 
 MechanismList.focus_path = 'mechanisms'
 
-function MechanismList.new(building)
+function MechanismList:init(building)
-    local self = {
+    self:init_fields{
-        links = {},
+        links = {}, selected = 1
-        selected = 1
     }
-    return mkinstance(MechanismList, self):init(building)
+    guidm.MenuOverlay.init(self)
+    self:fillList(building)
+    return self
 end
 
-function MechanismList:init(building)
+function MechanismList:fillList(building)
     local links = listMechanismLinks(building)
 
     links[1].viewport = self:getViewport()
@@ -71,7 +72,6 @@ function MechanismList:init(building)
 
     self.links = links
     self.selected = 1
-    return self
 end
 
 local colors = {
@@ -133,7 +133,7 @@ function MechanismList:onInput(keys)
         end
     elseif keys.SELECT_ALL then
         if self.selected > 1 then
-            self:init(self.links[self.selected].obj)
+            self:fillList(self.links[self.selected].obj)
         end
     elseif keys.SELECT then
         self:dismiss()
@@ -146,6 +146,6 @@ if dfhack.gui.getCurFocus() ~= 'dwarfmode/QueryBuilding/Some' then
     qerror("This script requires the main dwarfmode view in 'q' mode")
 end
 
-local list = MechanismList.new(df.global.world.selected_building)
+local list = mkinstance(MechanismList):init(df.global.world.selected_building)
 list:show()
 list:changeSelected(1)