3 files changed, 74 insertions, 14 deletions

diff --git a/src/nvim/buffer_defs.h b/src/nvim/buffer_defs.h
index d92d425ed5..5e9652792a 100644
--- a/src/nvim/buffer_defs.h
+++ b/src/nvim/buffer_defs.h
@@ -1226,6 +1226,8 @@ struct window_S {
 
   bool w_viewport_invalid;
   linenr_T w_viewport_last_topline;  // topline when the viewport was last updated
+  linenr_T w_viewport_last_topfill;  // topfill when the viewport was last updated
+  linenr_T w_viewport_last_skipcol;  // skipcol 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().
diff --git a/src/nvim/plines.c b/src/nvim/plines.c
index eb1ae540a4..46bbbeeb07 100644
--- a/src/nvim/plines.c
+++ b/src/nvim/plines.c
@@ -595,3 +595,52 @@ static int win_nolbr_chartabsize(chartabsize_T *cts, int *headp)
   }
   return n;
 }
+
+int64_t win_get_text_height(win_T *const wp, const linenr_T first, const linenr_T last,
+                            const int64_t start_vcol, const int64_t end_vcol)
+{
+  int width1 = 0;
+  int width2 = 0;
+  if (start_vcol >= 0 || end_vcol >= 0) {
+    width1 = wp->w_width_inner - win_col_off(wp);
+    width2 = width1 + win_col_off2(wp);
+    width1 = MAX(width1, 0);
+    width2 = MAX(width2, 0);
+  }
+
+  int64_t size = 0;
+  int64_t height_nofill = 0;
+  linenr_T lnum = first;
+
+  if (start_vcol >= 0) {
+    linenr_T lnum_next = lnum;
+    const bool folded = hasFoldingWin(wp, lnum, &lnum, &lnum_next, true, NULL);
+    height_nofill = folded ? 1 : plines_win_nofill(wp, lnum, false);
+    size += height_nofill;
+    const int64_t row_off = (start_vcol < width1 || width2 <= 0)
+                            ? 0
+                            : 1 + (start_vcol - width1) / width2;
+    size -= MIN(row_off, height_nofill);
+    lnum = lnum_next + 1;
+  }
+
+  while (lnum <= last) {
+    linenr_T lnum_next = lnum;
+    const bool folded = hasFoldingWin(wp, lnum, &lnum, &lnum_next, true, NULL);
+    height_nofill = folded ? 1 : plines_win_nofill(wp, lnum, false);
+    size += win_get_fill(wp, lnum) + height_nofill;
+    lnum = lnum_next + 1;
+  }
+
+  if (end_vcol >= 0) {
+    size -= height_nofill;
+    const int64_t row_off = end_vcol == 0
+                            ? 0
+                            : (end_vcol <= width1 || width2 <= 0)
+                              ? 1
+                              : 1 + (end_vcol - width1 + width2 - 1) / width2;
+    size += MIN(row_off, height_nofill);
+  }
+
+  return size;
+}
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 20e9dfec07..f5533db385 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -1037,22 +1037,31 @@ 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, scroll_delta);
+    int64_t delta = 0;
+    linenr_T last_topline = wp->w_viewport_last_topline;
+    int last_topfill = wp->w_viewport_last_topfill;
+    int64_t last_skipcol = wp->w_viewport_last_skipcol;
+    if (last_topline > line_count) {
+      delta -= last_topline - line_count;
+      last_topline = line_count;
+      last_topfill = 0;
+      last_skipcol = MAXCOL;
+    }
+    if (wp->w_topline < last_topline
+        || (wp->w_topline == last_topline && wp->w_skipcol < last_skipcol)) {
+      delta -= win_get_text_height(wp, wp->w_topline, last_topline, wp->w_skipcol, last_skipcol);
+    } else if ((wp->w_topline > last_topline && wp->w_topline <= line_count)
+               || (wp->w_topline == last_topline && wp->w_skipcol > last_skipcol)) {
+      delta += win_get_text_height(wp, last_topline, wp->w_topline, last_skipcol, wp->w_skipcol);
+    }
+    delta += last_topfill;
+    delta -= wp->w_topfill;
+    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, delta);
     wp->w_viewport_invalid = false;
     wp->w_viewport_last_topline = wp->w_topline;
+    wp->w_viewport_last_topfill = wp->w_topfill;
+    wp->w_viewport_last_skipcol = wp->w_skipcol;
   }
 }