T21a round 3 fix, after the cap

- TextInput records the selection each edit replaces, where every edit passes;
  the block editor's undo goes back to it, for typing, delete, cut and paste
- TextInput's layout geometry moves to forms/text/geometry.rs
- Reviews: T21a done, fixed after the cap, no fourth review

src/documents/blocks/edit.rs

@@ -174,14 +174,6 @@ impl BlockEditor {
         }
     }
 
-    /// Where a key went down, before the field acts on it: the start a typing burst's undo goes back to.
-    pub(crate) fn note_key(&mut self, key: u64, field: usize, cx: &Context<Self>) {
-        let selection = self.blocks[self.index(key)].fields[field]
-            .read(cx)
-            .selection();
-        self.typed_from = Some((key, field, selection));
-    }
-
     /// Records the document before a change to its blocks.
     pub(crate) fn before_change(&mut self, cx: &mut Context<Self>) {
         self.settle(true, cx);

src/documents/blocks/mod.rs

@@ -76,7 +76,7 @@ pub struct BlockEditor {
     current: Snapshot,
     /// The field that last held focus, as block and field.
     caret: Option<(u64, usize)>,
-    /// The selection a key went down on, before its field changed.
+    /// The selection the last edit in a field replaced.
     typed_from: Option<(u64, usize, std::ops::Range<usize>)>,
     pub(crate) people: Vec<SharedString>,
     pub(crate) diagram: Option<RenderDiagram>,
@@ -288,6 +288,8 @@ impl BlockEditor {
                             .insert(name.clone(), text);
                     }
                 }
+                let at = self.blocks[ix].fields.iter().position(|each| each == field);
+                self.typed_from = at.map(|at| (key, at, field.read(cx).edited_from()));
                 self.settle(true, cx);
             }
             InputEvent::Focus => {

src/documents/blocks/view.rs

@@ -114,7 +114,6 @@ impl BlockEditor {
                 cx.stop_propagation();
                 editor.on_submit(key, window, cx);
             }))
-            .capture_key_down(cx.listener(move |editor, _, _, cx| editor.note_key(key, ix, cx)))
             .on_mouse_down(MouseButton::Left, |_, _, cx| cx.stop_propagation())
             .child(field.clone());
         let slash = move |query: &str, at: usize, caret: usize| -> (Vec<Choice>, Pick) {