editor: Preserve scroll position when jumping from multibuffer (#9921)

This is a best-effort attempt, as the target offset from the top is just
an estimate; furthermore, this does not account for things like project
search header (which adds a bit of vertical offset by itself and is
removed once we jump into a buffer), but it still should improve the
situation quite a bit.

Fixes: #5296

Release Notes:

- Improved target selection when jumping from multibuffer; final
position in the buffer should more closely match the original position
of the cursor in the multibuffer.

crates/editor/src/scroll/autoscroll.rs

@@ -32,6 +32,10 @@ impl Autoscroll {
     pub fn focused() -> Self {
         Self::Strategy(AutoscrollStrategy::Focused)
     }
+    /// Scrolls so that the newest cursor is roughly an n-th line from the top.
+    pub fn top_relative(n: usize) -> Self {
+        Self::Strategy(AutoscrollStrategy::TopRelative(n))
+    }
 }
 
 #[derive(PartialEq, Eq, Default, Clone, Copy)]
@@ -43,6 +47,7 @@ pub enum AutoscrollStrategy {
     Focused,
     Top,
     Bottom,
+    TopRelative(usize),
 }
 
 impl AutoscrollStrategy {
@@ -178,6 +183,10 @@ impl Editor {
                 scroll_position.y = (target_bottom - visible_lines).max(0.0);
                 self.set_scroll_position_internal(scroll_position, local, true, cx);
             }
+            AutoscrollStrategy::TopRelative(lines) => {
+                scroll_position.y = target_top - lines as f32;
+                self.set_scroll_position_internal(scroll_position, local, true, cx);
+            }
         }
 
         self.scroll_manager.last_autoscroll = Some((