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/api/ui_events.in.h

@@ -114,7 +114,7 @@ void msg_set_pos(Integer grid, Integer row, Boolean scrolled, String sep_char)
   FUNC_API_SINCE(6) FUNC_API_COMPOSITOR_IMPL FUNC_API_CLIENT_IGNORE;
 
 void win_viewport(Integer grid, Window win, Integer topline, Integer botline, Integer curline,
-                  Integer curcol, Integer line_count)
+                  Integer curcol, Integer line_count, Integer scroll_delta)
   FUNC_API_SINCE(7) FUNC_API_CLIENT_IGNORE;
 
 void win_extmark(Integer grid, Window win, Integer ns_id, Integer mark_id, Integer row, Integer col)

src/nvim/buffer_defs.h

@@ -1222,6 +1222,7 @@ struct window_S {
   colnr_T w_valid_leftcol;          // last known w_leftcol
 
   bool w_viewport_invalid;
+  linenr_T w_viewport_last_topline;  // topline when the viewport was last updated
 
   // w_cline_height is the number of physical lines taken by the buffer line
   // that the cursor is on.  We use this to avoid extra calls to plines_win().

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;