documents: undo and redo leave a block only past its own history; typing lands after undo empties the document

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/forms/text/mod.rs

@@ -220,6 +220,14 @@ impl TextInput {
         &self.label
     }
 
+    /// A step left in that direction of its history.
+    pub(crate) fn can_step(&self, redo: bool) -> bool {
+        match redo {
+            true => self.history.can_redo(),
+            false => self.history.can_undo(),
+        }
+    }
+
     pub fn selection(&self) -> Range<usize> {
         self.selection.clone()
     }