vim-patch:9.0.0908: with 'smoothscroll' cursor may end up in wrong position

Problem:    With 'smoothscroll' cursor may end up in wrong position.
Solution:   Correct the computation of screen lines. (Yee Cheng Chin,
            closes vim/vim#11502)

https://github.com/vim/vim/commit/361895d2a15b4b0bbbb4c009261eab5b3d69ebf1

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

3 files changed, 96 insertions, 8 deletions

diff --git a/src/nvim/move.c b/src/nvim/move.c
index b0a4a3848a..abe908cfa0 100644
--- a/src/nvim/move.c
+++ b/src/nvim/move.c
@@ -1877,18 +1877,43 @@ void scroll_cursor_bot(int min_scroll, int set_topbot)
   // The lines of the cursor line itself are always used.
   used = plines_win_nofill(curwin, cln, true);
 
-  // If the cursor is below botline, we will at least scroll by the height
-  // of the cursor line.  Correct for empty lines, which are really part of
-  // botline.
+  // If the cursor is on or below botline, we will at least scroll by the
+  // height of the cursor line, which is "used".  Correct for empty lines,
+  // which are really part of botline.
   if (cln >= curwin->w_botline) {
     scrolled = used;
     if (cln == curwin->w_botline) {
       scrolled -= curwin->w_empty_rows;
     }
     min_scrolled = scrolled;
-    if (cln > curwin->w_botline && curwin->w_p_sms && curwin->w_p_wrap) {
-      for (linenr_T lnum = curwin->w_botline + 1; lnum <= cln; lnum++) {
-        min_scrolled += plines_win_nofill(curwin, lnum, true);
+    if (curwin->w_p_sms && curwin->w_p_wrap) {
+      // 'smoothscroll' and 'wrap' are set
+      if (cln > curwin->w_botline) {
+        // add screen lines below w_botline
+        for (linenr_T lnum = curwin->w_botline + 1; lnum <= cln; lnum++) {
+          min_scrolled += plines_win_nofill(curwin, lnum, true);
+        }
+      }
+
+      // Calculate how many screen lines the current top line of window
+      // occupies. If it is occupying more than the entire window, we
+      // need to scroll the additional clipped lines to scroll past the
+      // top line before we can move on to the other lines.
+      int top_plines = plines_win_nofill(curwin, curwin->w_topline, false);
+      int skip_lines = 0;
+      int width1 = curwin->w_width - curwin_col_off();
+      int width2 = width1 + curwin_col_off2();
+      // similar formula is used in curs_columns()
+      if (curwin->w_skipcol > width1) {
+        skip_lines += (curwin->w_skipcol - width1) / width2 + 1;
+      } else if (curwin->w_skipcol > 0) {
+        skip_lines = 1;
+      }
+
+      top_plines -= skip_lines;
+      if (top_plines > curwin->w_height) {
+        scrolled += (top_plines - curwin->w_height);
+        min_scrolled += (top_plines - curwin->w_height);
       }
     }
   }
diff --git a/test/functional/legacy/scroll_opt_spec.lua b/test/functional/legacy/scroll_opt_spec.lua
index a05470418f..6c8ca2cf97 100644
--- a/test/functional/legacy/scroll_opt_spec.lua
+++ b/test/functional/legacy/scroll_opt_spec.lua
@@ -410,7 +410,7 @@ describe('smoothscroll', function()
   it("adjusts the cursor position in a long line", function()
     screen:try_resize(40, 6)
     exec([[
-      call setline(1, ['one', 'two', 'Line' .. (' with lots of text'->repeat(30))])
+      call setline(1, ['one', 'two', 'Line' .. (' with lots of text'->repeat(30)) .. ' end', 'four'])
       set smoothscroll scrolloff=0
       normal 3G10|zt
     ]])
@@ -502,6 +502,45 @@ describe('smoothscroll', function()
       ith lots of text with lots of text with |
                                               |
     ]])
+    -- 'scrolloff' set to 0, move cursor down one line. Cursor should move properly,
+    -- and since this is a really long line, it will be put on top of the screen.
+    exec('set scrolloff=0')
+    feed('0j')
+    screen:expect([[
+      ^four                                    |
+      ~                                       |
+      ~                                       |
+      ~                                       |
+      ~                                       |
+                                              |
+    ]])
+    -- Repeat the step and move the cursor down again.
+    -- This time, use a shorter long line that is barely long enough to span more
+    -- than one window. Note that the cursor is at the bottom this time because
+    -- Vim prefers to do so if we are scrolling a few lines only.
+    exec("call setline(1, ['one', 'two', 'Line' .. (' with lots of text'->repeat(10)) .. ' end', 'four'])")
+    feed('3Gztj')
+    screen:expect([[
+      <<<th lots of text with lots of text wit|
+      h lots of text with lots of text with lo|
+      ts of text with lots of text with lots o|
+      f text with lots of text end            |
+      ^four                                    |
+                                              |
+    ]])
+    -- Repeat the step but this time start it when the line is smooth-scrolled by
+    -- one line. This tests that the offset calculation is still correct and
+    -- still end up scrolling down to the next line with cursor at bottom of
+    -- screen.
+    feed('3Gzt<C-E>j')
+    screen:expect([[
+      <<<th lots of text with lots of text wit|
+      h lots of text with lots of text with lo|
+      ts of text with lots of text with lots o|
+      f text with lots of text end            |
+      fou^r                                    |
+                                              |
+    ]])
   end)
 
   -- oldtest: Test_smoothscroll_one_long_line()
diff --git a/test/old/testdir/test_scroll_opt.vim b/test/old/testdir/test_scroll_opt.vim
index b9645898e8..8d1942e037 100644
--- a/test/old/testdir/test_scroll_opt.vim
+++ b/test/old/testdir/test_scroll_opt.vim
@@ -263,7 +263,7 @@ func Test_smoothscroll_wrap_long_line()
 
   let lines =<< trim END
       vim9script
-      setline(1, ['one', 'two', 'Line' .. (' with lots of text'->repeat(30))])
+      setline(1, ['one', 'two', 'Line' .. (' with lots of text'->repeat(30)) .. ' end', 'four'])
       set smoothscroll scrolloff=0
       normal 3G10|zt
   END
@@ -304,6 +304,30 @@ func Test_smoothscroll_wrap_long_line()
   call term_sendkeys(buf, "gj")
   call term_sendkeys(buf, "\<C-Y>")
   call VerifyScreenDump(buf, 'Test_smooth_long_9', {})
+
+  " 'scrolloff' set to 0, move cursor down one line.
+  " Cursor should move properly, and since this is a really long line, it will
+  " be put on top of the screen.
+  call term_sendkeys(buf, ":set scrolloff=0\<CR>")
+  call term_sendkeys(buf, "0j")
+  call VerifyScreenDump(buf, 'Test_smooth_long_10', {})
+
+  " Repeat the step and move the cursor down again.
+  " This time, use a shorter long line that is barely long enough to span more
+  " than one window. Note that the cursor is at the bottom this time because
+  " Vim prefers to do so if we are scrolling a few lines only.
+  call term_sendkeys(buf, ":call setline(1, ['one', 'two', 'Line' .. (' with lots of text'->repeat(10)) .. ' end', 'four'])\<CR>")
+  call term_sendkeys(buf, "3Gzt")
+  call term_sendkeys(buf, "j")
+  call VerifyScreenDump(buf, 'Test_smooth_long_11', {})
+
+  " Repeat the step but this time start it when the line is smooth-scrolled by
+  " one line. This tests that the offset calculation is still correct and
+  " still end up scrolling down to the next line with cursor at bottom of
+  " screen.
+  call term_sendkeys(buf, "3Gzt")
+  call term_sendkeys(buf, "\<C-E>j")
+  call VerifyScreenDump(buf, 'Test_smooth_long_12', {})
   
   call StopVimInTerminal(buf)
 endfunc