Add comment about the duplicate work in widget.height()

src/components/backlog.rs

@@ -130,6 +130,12 @@ impl Backlog {
         }) if *key == text_area.width => *height,
         prerender => {
           let widget = self.build_widget(item.content.clone(), 0);
+          // widget.height() needs to run the whole word-wrapping, which is almost as
+          // expensive as the real render.
+          // This is particularly wasteful, as the last widget.height() call here will
+          // duplicate the work we do in widget.render_overlap() later in the loop.
+          // Unfortunately I can't find a way to make it work because of the lifetimes
+          // involved.
           let expected_height = widget.height(text_area.width);
           *prerender = Some(PrerenderInner {
             key: text_area.width,