T21a round 1 fixes

- Enter over a selection drops it and splits at its start, one undo step
- undo and redo keep where the caret was and put it back
- a format unwraps only its own markers, so italic inside bold keeps the bold
- keys into a merged table cell land on the cell that shows
- undo rebuilds a block whose kind needs other fields, so styling comes back
- tests activate their window, as focus events need
- undo bookkeeping moves to blocks/edit.rs

src/documents/blocks/kind.rs

@@ -150,6 +150,16 @@ impl BlockKind {
         }
     }
 
+    /// Whether a field made for this kind serves `other` unchanged: the same highlighter and hint.
+    pub(crate) fn same_fields(&self, other: &Self) -> bool {
+        std::mem::discriminant(self) == std::mem::discriminant(other)
+            && match (self, other) {
+                (Self::Heading(one), Self::Heading(two)) => one == two,
+                (Self::Synced(one), Self::Synced(two)) => one == two,
+                _ => true,
+            }
+    }
+
     /// Prose: Enter splits it, Backspace at its start turns it back into text or joins it up.
     pub fn is_prose(&self) -> bool {
         matches!(