Say what may be done to a drawing, and default to nothing

`SizeDependence::{None, Internal, External}` becomes
`OnResize::{Scale, Translate, Redraw}`, which says what the retained path
may do rather than leaving the reader to work it out from a dependency.

`Redraw` is now the default, and that is the substance of this rather
than the naming. `Translate` was, and nothing opted into it: `SetSize`
reports one size and hands its child the whole box, so its pixels change
with the box and carrying them stretched a 100x100 rect across half the
window. A default that is only right for widgets that happen to qualify
is the same fault as an unchecked reuse flag.

`Translate` still does nothing, and now for the reason rather than the
one I gave before: `mov` translates perfectly well, but `ActiveData`'s
`region` is both the box a widget was given and the box its primitives
occupy, and `mov` remaps out of it. Keeping a drawing at its old size
while the box grows leaves those two disagreeing, and the next move
stretches it. Separating them is what the offset chain does.

Caught by rendering `tabs` against `main` rather than by a test, which is
the argument for keeping that check in the loop.

core/src/widget/mod.rs

@@ -15,20 +15,25 @@ pub use tag::*;
 pub use view::*;
 pub use widgets::*;
 
-/// How much of the box a widget was handed its drawing depends on, and so
-/// what has to change before it must be drawn again. Asked per axis, because
-/// wrapped text depends on the width it is offered and not on the height.
+/// What may be done to a widget's drawing when the box it was given changes
+/// on this axis, instead of drawing it again. Asked per axis, because wrapped
+/// text reads the width it is offered and not the height.
 #[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
-pub enum SizeDependence {
-    /// None of it: the box only says where the primitives go, so a new one is
-    /// written into them instead of drawn.
-    None,
-    /// Its own extent, whatever box that sits in. Reusable in any box that
-    /// leaves that extent unchanged, including a larger one it does not fill.
+pub enum OnResize {
+    /// Stretched to the new box, which is all its drawing ever was.
+    Scale,
+    /// Carried to the new box at the size it drew, which it keeps.
+    Translate,
+    /// Nothing doing: it draws again.
+    ///
+    /// The default, because it is the only answer that is right without
+    /// knowing anything about the widget. The other two are claims that the
+    /// drawing does not change when the box does, and a widget that inherited
+    /// such a claim by accident would be quietly wrong -- `SetSize` reports
+    /// one size and hands its child the whole box, so its pixels very much do
+    /// change.
     #[default]
-    Internal,
-    /// The extent of the box itself, used or not.
-    External,
+    Redraw,
 }
 
 pub trait Widget: Any {
@@ -42,8 +47,8 @@ pub trait Widget: Any {
         None
     }
 
-    fn size_dependence(&self, _axis: Axis) -> SizeDependence {
-        SizeDependence::Internal
+    fn on_resize(&self, _axis: Axis) -> OnResize {
+        OnResize::Translate
     }
 }
 
@@ -56,8 +61,8 @@ impl Widget for () {
         Some(Len::ZERO)
     }
 
-    fn size_dependence(&self, _axis: Axis) -> SizeDependence {
-        SizeDependence::None
+    fn on_resize(&self, _axis: Axis) -> OnResize {
+        OnResize::Scale
     }
 }