Snapshot: a document change puts the copied area away

Its page index would name another page after pages were deleted or moved.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/ui-egui/src/canvas.rs

@@ -1214,6 +1214,10 @@ impl DocView {
         if self.select_anchor.is_some_and(|a| a > last) {
             self.select_anchor = None;
         }
+        // A Snapshot area names a page by its index and a rectangle on it: after pages were
+        // deleted or moved it would outline, print and save a different page.
+        self.area_pick = None;
+        self.area_choice = None;
         self.goto = None;
         self.goto_point = None;
         self.zoom_anchor = None;

crates/ui-egui/tests/it/snapshot_area_tabs.rs

@@ -71,3 +71,13 @@ fn leaving_snapshot_clears_every_tab() {
     assert!(h.state().views[1].area_pick.is_none(), "the tab on screen");
     assert!(h.state().views[0].area_pick.is_none(), "the other tab");
 }
+
+/// A change to the page list leaves no area behind: its page index would name another page.
+#[test]
+fn a_document_change_puts_the_area_away() {
+    let mut h = harness();
+    h.state_mut().views[0].area_pick = Some((0, [10.0, 130.0, 150.0, 170.0]));
+    let info = h.state().session.get(h.state().views[0].id).unwrap().info.clone();
+    h.state_mut().views[0].document_changed(&info);
+    assert!(h.state().views[0].area_pick.is_none());
+}