Don't clip elements of non-clipping div.

The default `Div` is non-clipping. However, when children changed we
clipped them. This meant that the initial draw would happily draw
elements outside the borders, but further updates would not.

src/element.toit

@@ -274,6 +274,15 @@ class Div extends Element:
     if change-tracker and x and y and w and h:
       change-tracker.child-invalidated x y w h
 
+  child-invalidated x/int y/int w/int h/int --clip/bool=false -> none:
+    if clip:
+      super x y w h
+    else:
+      if change-tracker:
+        x2 := x_ + x
+        y2 := y_ + y
+        change-tracker.child-invalidated x2 y2 w h
+
   w -> int?: return w_
   h -> int?: return h_
 
@@ -599,6 +608,9 @@ class ClippingDiv_ extends Div:
       extent --x=x --y=y --w=w --h=h: | outer-x outer-y outer-w outer-h |
         change-tracker.child-invalidated outer-x outer-y outer-w outer-h
 
+  child-invalidated x/int y/int w/int h/int -> none:
+    super x y w h --clip
+
   static is-all-transparent opacity -> bool:
     if opacity is not ByteArray: return false
     return opacity.size == 1 and opacity[0] == 0

tests/div-visualized.toit

@@ -0,0 +1,37 @@
+// Copyright (C) 2023 Toitware ApS.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the TESTS_LICENSE file.
+
+// Tests some simple vertical sliders where there is a movable boundary between
+// two different backgrounds.
+
+import expect show *
+import font show *
+import pixel-display show *
+import .png-visualizer
+
+main args:
+  if args.size != 1:
+    print "Usage: script.toit png-basename"
+    exit 1
+  WIDTH ::= 220
+  HEIGHT ::= 140
+  driver := TrueColorPngVisualizer WIDTH HEIGHT args[0] --outline=0x4040ff
+  display := PixelDisplay.true-color driver
+  display.background = 0x808080
+
+  sans10 := Font.get "sans10"
+  label := Label --x=0 --y=0 --text="foo" --font=sans10
+  div := Div --x=30 --y=30 --w=(WIDTH - 60) --h=(HEIGHT - 60) [
+    // Since this isn't a clipping div, the label is drawn, even though it is
+    // outside the div.
+    label
+  ]
+  display.add div
+
+  display.draw
+
+  label.text = "bar"
+  display.draw
+
+  driver.write-png