Allow to change labels when they aren't mounted.

src/element.toit

@@ -74,7 +74,7 @@ abstract class Element implements Window:
   abstract invalidate -> none
 
   is-mounted -> bool:
-    return change-tracker and change-tracker.is-mounted
+    return change-tracker != null and change-tracker.is-mounted
 
   /**
   Finds an Element in the tree with the given id.
@@ -484,7 +484,11 @@ class Label extends Element implements ColoredElement:
 
   text= value/string -> none:
     if value == text_: return
-    if orientation_ == ORIENTATION-0 and change-tracker and x and y:
+    if not is-mounted:
+      text_ = value
+      left_ = null  // Trigger recalculation.
+      return
+    if orientation_ == ORIENTATION-0 and change-tracker and x and y and font_ and text_:
       text-get-bounding-boxes_ text_ value alignment_ font_: | old/TextExtent_ new/TextExtent_ |
         change-tracker.child-invalidated (x_ + old.x) (y_ + old.y) old.w old.h
         change-tracker.child-invalidated (x_ + new.x) (y_ + new.y) new.w new.h

tests/label-unmounted-visualized.toit

@@ -0,0 +1,50 @@
+// 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 for Label that the change box is smaller when we only
+// change part of the text.
+
+import bitmap show *
+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
+  driver := SeveralColorPngVisualizer 192 96 args[0] --outline=SEVERAL-WHITE
+  display := PixelDisplay.several-color driver
+  display.background = SEVERAL-DARK-GRAY
+
+  sans10 := Font.get "sans10"
+  sans12 := Font.get "sans12"
+
+  element-text := Label --x=30 --y=20 --color=SEVERAL-ORANGE --text="to be set"
+  element-text-2 := Label --x=180 --y=50 --color=SEVERAL-ORANGE --text="to be set" --font=sans12 --alignment=ALIGN-RIGHT
+  element-text-3 := Label --x=96 --y=80 --color=SEVERAL-ORANGE --text="T 123 For the win" --font=sans10 --alignment=ALIGN-CENTER
+
+  element-text.text = "Testing 123"
+  element-text.font = sans10
+  element-text-2.text = "123 Testing"
+  element-text-2.font = sans10
+  display.add element-text
+  display.add element-text-2
+  display.add element-text-3
+  display.draw
+
+  display.remove-all
+
+  element-text.label = "Testing 42"
+  element-text-2.label = "42 Testing"
+  // The "MM" has the same pixel width as the "123" above, so we can test the
+  // case where the ends are unchanged, but the middle changes.
+  element-text-3.label = "T MM For the win"
+  display.add element-text
+  display.add element-text-2
+  display.add element-text-3
+  display.draw
+
+  driver.write-png