feat(ui): add scroll_delta to win_viewport event #19270

scroll_delta contains how much the top line of a window moved since the
last time win_viewport was emitted. It is expected to be used to
implement smooth scrolling. For this purpose it only counts "virtual" or
"displayed" so folds should count as one line. Because of this it
adds extra information that cannot be computed from the topline
parameter.

Fixes #19227

src/nvim/window.c

@@ -993,10 +993,22 @@ void ui_ext_win_viewport(win_T *wp)
       // interact with incomplete final line? Diff filler lines?
       botline = wp->w_buffer->b_ml.ml_line_count;
     }
+    int scroll_delta = 0;
+    if (wp->w_viewport_last_topline > line_count) {
+      scroll_delta -= wp->w_viewport_last_topline - line_count;
+      wp->w_viewport_last_topline = line_count;
+    }
+    if (wp->w_topline < wp->w_viewport_last_topline) {
+      scroll_delta -= plines_m_win(wp, wp->w_topline, wp->w_viewport_last_topline - 1);
+    } else if (wp->w_topline > wp->w_viewport_last_topline
+               && wp->w_topline <= line_count) {
+      scroll_delta += plines_m_win(wp, wp->w_viewport_last_topline, wp->w_topline - 1);
+    }
     ui_call_win_viewport(wp->w_grid_alloc.handle, wp->handle, wp->w_topline - 1,
                          botline, wp->w_cursor.lnum - 1, wp->w_cursor.col,
-                         line_count);
+                         line_count, scroll_delta);
     wp->w_viewport_invalid = false;
+    wp->w_viewport_last_topline = wp->w_topline;
   }
 }
 
@@ -5038,6 +5050,7 @@ static win_T *win_alloc(win_T *after, bool hidden)
   new_wp->w_floating = 0;
   new_wp->w_float_config = FLOAT_CONFIG_INIT;
   new_wp->w_viewport_invalid = true;
+  new_wp->w_viewport_last_topline = 1;
 
   new_wp->w_ns_hl = -1;