Constrain offered boxes with independent widget size bounds

core/src/widget/widgets.rs

@@ -145,6 +145,16 @@ impl Widgets {
         self.needs_redraw.insert(id);
     }
 
+    /// Changes the preferred length without changing its bounds.
+    pub fn set_len(&mut self, id: impl IdLike, axis: Axis, len: impl Into<crate::SizeRequest>) {
+        let id = id.id();
+        let rule = SizeRule {
+            request: Some(len.into()),
+            bound: self.size_rules(id)[axis].bound,
+        };
+        self.set_size_rule(id, axis, rule);
+    }
+
     /// Puts a floor under this widget's length on one axis, keeping a cap it
     /// already had. See [`SizeRule::at_least`].
     pub fn set_min_len(&mut self, id: impl IdLike, axis: Axis, min: Len) {