Update buffer reload to retain cursor position when possible

src/buffer/mod.rs

@@ -439,6 +439,9 @@ impl Buffer {
                     self.id = buf.id;
                     self.syntax_definition = buf.syntax_definition;
                     self.change_callback = buf.change_callback;
+                    if !self.cursor.move_to(*buf.cursor) {
+                        self.cursor.move_to(Position{ line: buf.cursor.line, offset: 0 });
+                    }
                 },
                 Err(e) => return Err(e),
             }
@@ -510,6 +513,57 @@ mod tests {
         assert!(buffer.syntax_definition.is_some());
     }
 
+    #[test]
+    fn reload_retains_full_position_when_possible() {
+        // Load a buffer with some data and modify it.
+        let file_path = Path::new("tests/sample/file");
+        let mut buffer = Buffer::from_file(file_path).unwrap();
+
+        // Move to a position that will exist after reload.
+        buffer.cursor.move_to(Position { line: 0, offset: 3 });
+
+        // Reload the buffer
+        buffer.reload().unwrap();
+
+        // Verify that the position is retained.
+        assert_eq!(*buffer.cursor, Position { line: 0, offset: 3 });
+    }
+
+    #[test]
+    fn reload_retains_position_line_when_possible() {
+        // Load a buffer with some data and modify it.
+        let file_path = Path::new("tests/sample/file");
+        let mut buffer = Buffer::from_file(file_path).unwrap();
+
+        // Move to an in-memory-only position whose line
+        // is also available in the on-disk version.
+        buffer.insert("amp\neditor");
+        buffer.cursor.move_to(Position { line: 1, offset: 1 });
+
+        // Reload the buffer
+        buffer.reload().unwrap();
+
+        // Verify that the position is set to the start of the same line.
+        assert_eq!(*buffer.cursor, Position { line: 1, offset: 0 });
+    }
+
+    #[test]
+    fn reload_discards_position_when_impossible() {
+        // Load a buffer with some data and modify it.
+        let file_path = Path::new("tests/sample/file");
+        let mut buffer = Buffer::from_file(file_path).unwrap();
+
+        // Move to a truly in-memory-only position.
+        buffer.insert("\namp\neditor");
+        buffer.cursor.move_to(Position { line: 2, offset: 1 });
+
+        // Reload the buffer
+        buffer.reload().unwrap();
+
+        // Verify that the position is discarded.
+        assert_eq!(*buffer.cursor, Position::new());
+    }
+
     #[test]
     fn reload_calls_change_callback_with_zero_position() {
         // Load a buffer with some data and modify it.