Avoid losing focus when block decorations go offscreen (#14815)

Release Notes:

- Fixed a bug that caused focus to be lost when renames and inline
assists were scrolled offscreen.

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/multi_buffer/src/multi_buffer.rs

@@ -261,7 +261,7 @@ pub struct ExcerptRange<T> {
 }
 
 #[derive(Clone, Debug, Default)]
-struct ExcerptSummary {
+pub struct ExcerptSummary {
     excerpt_id: ExcerptId,
     /// The location of the last [`Excerpt`] being summarized
     excerpt_locator: Locator,
@@ -3744,6 +3744,21 @@ impl MultiBufferSnapshot {
         Some(&self.excerpt(excerpt_id)?.buffer)
     }
 
+    pub fn range_for_excerpt<'a, T: sum_tree::Dimension<'a, ExcerptSummary>>(
+        &'a self,
+        excerpt_id: ExcerptId,
+    ) -> Option<Range<T>> {
+        let mut cursor = self.excerpts.cursor::<(Option<&Locator>, T)>();
+        let locator = self.excerpt_locator_for_id(excerpt_id);
+        if cursor.seek(&Some(locator), Bias::Left, &()) {
+            let start = cursor.start().1.clone();
+            let end = cursor.end(&()).1;
+            Some(start..end)
+        } else {
+            None
+        }
+    }
+
     fn excerpt(&self, excerpt_id: ExcerptId) -> Option<&Excerpt> {
         let mut cursor = self.excerpts.cursor::<Option<&Locator>>();
         let locator = self.excerpt_locator_for_id(excerpt_id);