feat(ui): one mounted tree instead of a screen object per build

Chrome and the app now share a tree, so a screen is no longer something an
app constructs and holds: ui.mount() takes the function that builds the
whole thing and ui.rebuild() runs it again. Building a node after layout
is refused rather than silently resetting the arena under the panel.

lua/test/ui.lua

@@ -186,27 +186,30 @@ local function handler(name)
   end
 end
 
-local first = ui.button {
-  label = "one",
-  on_enter = handler "enter",
-  on_exit = handler "exit",
-  on_click = handler "click",
-}
-local second = ui.button {
-  label = "two",
-  on_enter = handler "enter",
-  on_exit = handler "exit",
-  on_click = handler "click",
-}
-local screen = ui.screen(ui.box { row = true, first, second })
+local first, second
+ui.mount(function()
+  first = ui.button {
+    label = "one",
+    on_enter = handler "enter",
+    on_exit = handler "exit",
+    on_click = handler "click",
+  }
+  second = ui.button {
+    label = "two",
+    on_enter = handler "enter",
+    on_exit = handler "exit",
+    on_click = handler "click",
+  }
+  return ui.box { row = true, first, second }
+end)
 
-assert(screen:down(10, 10))
+assert(ui.down(10, 10))
 assert(node.isPressed(first))
-assert(screen:move(95, 10))
+assert(ui.move(95, 10))
 assert(not node.isPressed(first))
-assert(screen:move(10, 10))
+assert(ui.move(10, 10))
 assert(node.isPressed(first))
-assert(screen:up(10, 10))
+assert(ui.up(10, 10))
 assert(not node.isPressed(first))
 
 local expectedTouch = { "enter", "exit", "enter", "exit", "click" }
@@ -218,16 +221,16 @@ for index, name in ipairs(expectedTouch) do
 end
 
 events = {}
-assert(screen:button("right", true))
-assert(screen:button("right", false))
+assert(ui.buttonPress("right", true))
+assert(ui.buttonPress("right", false))
 assert(node.getFocus() == first)
-assert(screen:button("right", true))
+assert(ui.buttonPress("right", true))
 assert(node.getFocus() == second)
-assert(screen:button("confirm", true))
+assert(ui.buttonPress("confirm", true))
 assert(node.isPressed(second))
-assert(screen:button("confirm", false))
+assert(ui.buttonPress("confirm", false))
 assert(not node.isPressed(second))
-assert(screen:button("back", true) == false)
+assert(ui.buttonPress("back", true) == false)
 
 local expectedButtons = {
   { "enter", first },
@@ -246,7 +249,7 @@ assert(ui.setTheme "mono" == true)
 assert(ui.getTheme() == "mono" and #invalidated == before + 1)
 assert(cleared == ui.theme.background)
 
-screen:draw()
+ui.draw()
 
 -- 320x480 portrait: two columns, and the reserve comes off the height budget.
 local side, columns = ui.cardSide(5, 12, 8)
@@ -265,21 +268,32 @@ local pressedCalls = {}
 node.setPressed = function(_, on)
   pressedCalls[#pressedCalls + 1] = on
 end
-local own = ui.custom { h = 20, press_style = false, on_click = function() end }
-local styled = ui.button { h = 20, label = "ok", on_click = function() end }
-local board = ui.screen(ui.box { own, styled })
+local own, styled
+ui.mount(function()
+  own = ui.custom { h = 20, press_style = false, on_click = function() end }
+  styled = ui.button { h = 20, label = "ok", on_click = function() end }
+  return ui.box { own, styled }
+end)
 local target
 node.hit = function()
   return target
 end
 
 target = own
-board:down(0, 0)
-board:up(0, 0)
+ui.down(0, 0)
+ui.up(0, 0)
 assert(#pressedCalls == 0, "a self-painting widget is not styled on press")
 
 target = styled
-board:down(0, 0)
+ui.down(0, 0)
 assert(pressedCalls[1] == true, "an ordinary widget still gets its pressed style")
 
+-- Building outside a rebuild would reset the arena under the screen on the panel.
+local built = pcall(ui.button, { label = "stray" })
+assert(not built, "a node built after layout is refused")
+local rebuilt = false
+ui.rebuild()
+rebuilt = true
+assert(rebuilt and ui.down(0, 0), "a rebuild replaces the screen and keeps dispatch live")
+
 print "ok"