vim-patch:9.0.0900: cursor moves too far with 'smoothscroll'

Problem:    Cursor moves too far with 'smoothscroll'.
Solution:   Only move as far as really needed. (Yee Cheng Chin, closes vim/vim#11504)

https://github.com/vim/vim/commit/81ba26e9de24ca6b1c05b6ec03e53b21793f1a4b

Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>

1 files changed, 36 insertions, 9 deletions

diff --git a/src/nvim/move.c b/src/nvim/move.c
index 8113331c62..b0a4a3848a 100644
--- a/src/nvim/move.c
+++ b/src/nvim/move.c
@@ -164,6 +164,22 @@ static void redraw_for_cursorcolumn(win_T *wp)
   }
 }
 
+/// Calculates how much overlap the smoothscroll marker "<<<" overlaps with
+/// buffer text for curwin.
+/// Parameter "extra2" should be the padding on the 2nd line, not the first
+/// line.
+/// Returns the number of columns of overlap with buffer text, excluding the
+/// extra padding on the ledge.
+static int smoothscroll_marker_overlap(win_T *wp, int extra2)
+{
+  // We don't draw the <<< marker when in showbreak mode, thus no need to
+  // account for it.  See grid_put_linebuf().
+  if (*get_showbreak_value(wp) != NUL) {
+    return 0;
+  }
+  return extra2 > 3 ? 0 : 3 - extra2;
+}
+
 /// Set wp->s_skipcol to zero and redraw later if needed.
 static void reset_skipcol(win_T *wp)
 {
@@ -239,10 +255,12 @@ void update_topline(win_T *wp)
       } else if (wp->w_skipcol > 0 && wp->w_cursor.lnum == wp->w_topline) {
         colnr_T vcol;
 
-        // check the cursor position is visible.  Add 3 for the ">>>"
-        // displayed in the top-left.
+        // Check that the cursor position is visible.  Add columns for the
+        // smoothscroll marker "<<<" displayed in the top-left if needed.
         getvvcol(wp, &wp->w_cursor, &vcol, NULL, NULL);
-        if (wp->w_skipcol + 3 >= vcol) {
+        int smoothscroll_overlap = smoothscroll_marker_overlap(wp,
+                                                               win_col_off(wp) - win_col_off2(wp));
+        if (wp->w_skipcol + smoothscroll_overlap > vcol) {
           check_topline = true;
         }
       }
@@ -1387,12 +1405,21 @@ bool scrollup(long line_count, int byfold)
   }
 
   if (curwin->w_cursor.lnum == curwin->w_topline && do_sms && curwin->w_skipcol > 0) {
-    int width1 = curwin->w_width - curwin_col_off();
-    int width2 = width1 + curwin_col_off2();
+    int col_off = curwin_col_off();
+    int col_off2 = curwin_col_off2();
+
+    int width1 = curwin->w_width - col_off;
+    int width2 = width1 + col_off2;
+    int extra2 = col_off - col_off2;
     long so = get_scrolloff_value(curwin);
     long scrolloff_cols = so == 0 ? 0 : width1 + (so - 1) * width2;
     int space_cols = (curwin->w_height - 1) * width2;
 
+    // If we have non-zero scrolloff, just ignore the <<< marker as we are
+    // going past it anyway.
+    int smoothscroll_overlap = scrolloff_cols != 0 ? 0 :
+                               smoothscroll_marker_overlap(curwin, extra2);
+
     // Make sure the cursor is in a visible part of the line, taking
     // 'scrolloff' into account, but using screen lines.
     // If there are not enough screen lines put the cursor in the middle.
@@ -1400,13 +1427,13 @@ bool scrollup(long line_count, int byfold)
       scrolloff_cols = space_cols / 2;
     }
     validate_virtcol();
-    if (curwin->w_virtcol < curwin->w_skipcol + 3 + scrolloff_cols) {
+    if (curwin->w_virtcol < curwin->w_skipcol + smoothscroll_overlap + scrolloff_cols) {
       colnr_T col = curwin->w_virtcol;
 
       if (col < width1) {
         col += width1;
       }
-      while (col < curwin->w_skipcol + 3 + scrolloff_cols) {
+      while (col < curwin->w_skipcol + smoothscroll_overlap + scrolloff_cols) {
         col += width2;
       }
       curwin->w_curswant = col;
@@ -2180,9 +2207,9 @@ void cursor_correct(void)
   curwin->w_viewport_invalid = true;
 }
 
-// move screen 'count' pages up or down and update screen
+// Move screen "count" pages up or down and update screen.
 //
-// return FAIL for failure, OK otherwise
+// Return FAIL for failure, OK otherwise.
 int onepage(Direction dir, long count)
 {
   long n;