fix(tui): use updated v_offset after params_changed in ensure_viewport_cache (#24)

The loading branch of ensure_viewport_cache captured v_offset before the
params_changed block, which could reassign self.v_offset. This caused the
viewport to use a stale offset when loading + width/format changed together.

Remove the stale local variable and read self.v_offset directly, consistent
with the non-loading branch. Add regression test.

crates/tui/src/app.rs

@@ -246,7 +246,6 @@ impl App {
     /// Returns (start_logical, offset_in_line) for rendering.
     pub(crate) fn ensure_viewport_cache(&mut self, width: usize) -> (usize, usize) {
         let viewport_height = self.content_height as usize;
-        let v_offset = self.v_offset;
 
         if !self.is_loaded() || width == 0 || viewport_height == 0 {
             return (0, 0);
@@ -268,7 +267,7 @@ impl App {
 
         // Find start logical line from v_offset
         let (start_logical, offset_in_line) = if self.is_loading() {
-            (v_offset.min(self.total_lines().saturating_sub(1)), self.v_sub_offset)
+            (self.v_offset.min(self.total_lines().saturating_sub(1)), self.v_sub_offset)
         } else {
             self.find_logical_line_at_visual_row(self.v_offset, width)
         };
@@ -2538,6 +2537,42 @@ plain text line
         assert!(result.is_ok());
     }
 
+    /// Regression test for issue #24:
+    /// ensure_viewport_cache must use the updated self.v_offset after params_changed
+    /// recalculates it, not a stale local captured before the change block.
+    #[test]
+    fn test_loading_viewport_cache_uses_updated_v_offset_on_params_changed() {
+        let content: String = (0..200).map(|i| format!("line{i}\n")).collect();
+        let path = make_temp_file(&content);
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.load_file(path.to_str().unwrap()).unwrap();
+            assert!(app.is_loading(), "should be in Loading state");
+
+            app.content_height = 10;
+            app.ensure_viewport_cache(80);
+
+            app.v_offset = 90;
+            app.cursor_line = 100;
+
+            let new_width = 40;
+            app.ensure_viewport_cache(new_width);
+
+            let recomputed_offset = app.v_offset;
+            assert_ne!(
+                recomputed_offset, 90,
+                "v_offset should have been recalculated by params_changed block, still 90"
+            );
+
+            assert_eq!(
+                app.viewport_cache.logical_start, recomputed_offset.min(app.total_lines().saturating_sub(1)),
+                "logical_start should match the updated v_offset"
+            );
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
     fn install_vhi(app: &mut App, heights: &[usize]) {
         let width = app.get_content_width();
         let json_format = app.json_format;