fix(api): fix inconsistent behavior of topline touched in recent refactor

The change in #24824 0081549 was not a regression, however it was an
incomplete change. Unfortunately some common plugins come to depend on
this exising self-inconsistent behavior. These plugins are going to need
to update for 0.10

nvim_buf_set_lines used to NOT adjust the topline correctly if a buffer
was displayed in just one window. However, if displayed in multiple
windows, it was correctly adjusted for any window not deemed the
current window for the buffer (which could be an arbitrary choice if the
buffer was not already current, as noted in the last rafactor)

This fixes so that all windows have their topline adjusted. The added
tests show this behavior, which should be the reasonable one.

3 files changed, 289 insertions, 21 deletions

diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index 775b6e8ea7..d36f0dd050 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -472,9 +472,11 @@ void nvim_buf_set_lines(uint64_t channel_id, Buffer buffer, Integer start, Integ
                  kExtmarkUndo);
 
   changed_lines(buf, (linenr_T)start, 0, (linenr_T)end, (linenr_T)extra, true);
-  if (curwin->w_buffer == buf) {
-    // mark_adjust_buf handles non-current windows
-    fix_cursor(curwin, (linenr_T)start, (linenr_T)end, (linenr_T)extra);
+
+  FOR_ALL_TAB_WINDOWS(tp, win) {
+    if (win->w_buffer == buf) {
+      fix_cursor(win, (linenr_T)start, (linenr_T)end, (linenr_T)extra);
+    }
   }
 
 end:
@@ -710,7 +712,7 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
   // changed range, and move any in the remainder of the buffer.
   // Do not adjust any cursors. need to use column-aware logic (below)
   mark_adjust_buf(buf, (linenr_T)start_row, (linenr_T)end_row, MAXLNUM, (linenr_T)extra,
-                  true, false, kExtmarkNOOP);
+                  true, true, kExtmarkNOOP);
 
   extmark_splice(buf, (int)start_row - 1, (colnr_T)start_col,
                  (int)(end_row - start_row), col_extent, old_byte,
diff --git a/src/nvim/mark.c b/src/nvim/mark.c
index 2e08a6f591..584a6c5827 100644
--- a/src/nvim/mark.c
+++ b/src/nvim/mark.c
@@ -1121,7 +1121,7 @@ void ex_changes(exarg_T *eap)
 void mark_adjust(linenr_T line1, linenr_T line2, linenr_T amount, linenr_T amount_after,
                  ExtmarkOp op)
 {
-  mark_adjust_buf(curbuf, line1, line2, amount, amount_after, true, true, op);
+  mark_adjust_buf(curbuf, line1, line2, amount, amount_after, true, false, op);
 }
 
 // mark_adjust_nofold() does the same as mark_adjust() but without adjusting
@@ -1132,11 +1132,11 @@ void mark_adjust(linenr_T line1, linenr_T line2, linenr_T amount, linenr_T amoun
 void mark_adjust_nofold(linenr_T line1, linenr_T line2, linenr_T amount, linenr_T amount_after,
                         ExtmarkOp op)
 {
-  mark_adjust_buf(curbuf, line1, line2, amount, amount_after, false, true, op);
+  mark_adjust_buf(curbuf, line1, line2, amount, amount_after, false, false, op);
 }
 
 void mark_adjust_buf(buf_T *buf, linenr_T line1, linenr_T line2, linenr_T amount,
-                     linenr_T amount_after, bool adjust_folds, bool adj_cursor, ExtmarkOp op)
+                     linenr_T amount_after, bool adjust_folds, bool by_api, ExtmarkOp op)
 {
   int fnum = buf->b_fnum;
   linenr_T *lp;
@@ -1243,7 +1243,7 @@ void mark_adjust_buf(buf_T *buf, linenr_T line1, linenr_T line2, linenr_T amount
 
       // topline and cursor position for windows with the same buffer
       // other than the current window
-      if (win != curwin) {
+      if (win != curwin || by_api) {
         if (win->w_topline >= line1 && win->w_topline <= line2) {
           if (amount == MAXLNUM) {                  // topline is deleted
             if (line1 <= 1) {
@@ -1261,21 +1261,21 @@ void mark_adjust_buf(buf_T *buf, linenr_T line1, linenr_T line2, linenr_T amount
           win->w_topline += amount_after;
           win->w_topfill = 0;
         }
-        if (adj_cursor) {
-          if (win->w_cursor.lnum >= line1 && win->w_cursor.lnum <= line2) {
-            if (amount == MAXLNUM) {         // line with cursor is deleted
-              if (line1 <= 1) {
-                win->w_cursor.lnum = 1;
-              } else {
-                win->w_cursor.lnum = line1 - 1;
-              }
-              win->w_cursor.col = 0;
-            } else {                      // keep cursor on the same line
-              win->w_cursor.lnum += amount;
+      }
+      if (win != curwin && !by_api) {
+        if (win->w_cursor.lnum >= line1 && win->w_cursor.lnum <= line2) {
+          if (amount == MAXLNUM) {         // line with cursor is deleted
+            if (line1 <= 1) {
+              win->w_cursor.lnum = 1;
+            } else {
+              win->w_cursor.lnum = line1 - 1;
             }
-          } else if (amount_after && win->w_cursor.lnum > line2) {
-            win->w_cursor.lnum += amount_after;
+            win->w_cursor.col = 0;
+          } else {                      // keep cursor on the same line
+            win->w_cursor.lnum += amount;
           }
+        } else if (amount_after && win->w_cursor.lnum > line2) {
+          win->w_cursor.lnum += amount_after;
         }
       }
 
diff --git a/test/functional/api/buffer_spec.lua b/test/functional/api/buffer_spec.lua
index 4784c0a9dd..fda2ea17b8 100644
--- a/test/functional/api/buffer_spec.lua
+++ b/test/functional/api/buffer_spec.lua
@@ -137,6 +137,139 @@ describe('api/buf', function()
       -- it's impossible to get out-of-bounds errors for an unloaded buffer
       eq({}, buffer('get_lines', bufnr, 8888, 9999, 1))
     end)
+
+    describe('handles topline', function()
+      local screen
+      before_each(function()
+        screen = Screen.new(20, 12)
+        screen:set_default_attr_ids {
+          [1] = {bold = true, foreground = Screen.colors.Blue1};
+          [2] = {reverse = true, bold = true};
+          [3] = {reverse = true};
+        }
+        screen:attach()
+        meths.buf_set_lines(0, 0, -1, 1, {"aaa", "bbb", "ccc", "ddd", "www", "xxx", "yyy", "zzz"})
+        meths.set_option_value('modified', false, {})
+      end)
+
+      it('of current window', function()
+        local win = meths.get_current_win()
+        local buf = meths.get_current_buf()
+
+        command('new | wincmd w')
+        meths.win_set_cursor(win, {8,0})
+
+        screen:expect{grid=[[
+                              |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {3:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          ^zzz                 |
+          {2:[No Name]           }|
+                              |
+        ]]}
+        meths.buf_set_lines(buf, 0, 2, true, {"aaabbb"})
+
+        screen:expect{grid=[[
+                              |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {3:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          ^zzz                 |
+          {2:[No Name] [+]       }|
+                              |
+        ]]}
+      end)
+
+      it('of non-current window', function()
+        local win = meths.get_current_win()
+        local buf = meths.get_current_buf()
+
+        command('new')
+        meths.win_set_cursor(win, {8,0})
+
+        screen:expect{grid=[[
+          ^                    |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {2:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name]           }|
+                              |
+        ]]}
+
+        meths.buf_set_lines(buf, 0, 2, true, {"aaabbb"})
+        screen:expect{grid=[[
+          ^                    |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {2:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name] [+]       }|
+                              |
+        ]]}
+      end)
+
+      it('of split windows with same buffer', function()
+        local win = meths.get_current_win()
+        local buf = meths.get_current_buf()
+
+        command('split')
+        meths.win_set_cursor(win, {8,0})
+        meths.win_set_cursor(0, {1,0})
+
+        screen:expect{grid=[[
+          ^aaa                 |
+          bbb                 |
+          ccc                 |
+          ddd                 |
+          www                 |
+          {2:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name]           }|
+                              |
+        ]]}
+        meths.buf_set_lines(buf, 0, 2, true, {"aaabbb"})
+
+        screen:expect{grid=[[
+          ^aaabbb              |
+          ccc                 |
+          ddd                 |
+          www                 |
+          xxx                 |
+          {2:[No Name] [+]       }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name] [+]       }|
+                              |
+        ]]}
+      end)
+    end)
   end)
 
   describe('deprecated: {get,set,del}_line', function()
@@ -659,6 +792,139 @@ describe('api/buf', function()
       ]])
       eq({'one', 'two'}, get_lines(0, 2, true))
     end)
+
+    describe('handles topline', function()
+      local screen
+      before_each(function()
+        screen = Screen.new(20, 12)
+        screen:set_default_attr_ids {
+          [1] = {bold = true, foreground = Screen.colors.Blue1};
+          [2] = {reverse = true, bold = true};
+          [3] = {reverse = true};
+        }
+        screen:attach()
+        meths.buf_set_lines(0, 0, -1, 1, {"aaa", "bbb", "ccc", "ddd", "www", "xxx", "yyy", "zzz"})
+        meths.set_option_value('modified', false, {})
+      end)
+
+      it('of current window', function()
+        local win = meths.get_current_win()
+        local buf = meths.get_current_buf()
+
+        command('new | wincmd w')
+        meths.win_set_cursor(win, {8,0})
+
+        screen:expect{grid=[[
+                              |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {3:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          ^zzz                 |
+          {2:[No Name]           }|
+                              |
+        ]]}
+        meths.buf_set_text(buf, 0,3, 1,0, {"X"})
+
+        screen:expect{grid=[[
+                              |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {3:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          ^zzz                 |
+          {2:[No Name] [+]       }|
+                              |
+        ]]}
+      end)
+
+      it('of non-current window', function()
+        local win = meths.get_current_win()
+        local buf = meths.get_current_buf()
+
+        command('new')
+        meths.win_set_cursor(win, {8,0})
+
+        screen:expect{grid=[[
+          ^                    |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {2:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name]           }|
+                              |
+        ]]}
+
+        meths.buf_set_text(buf, 0,3, 1,0, {"X"})
+        screen:expect{grid=[[
+          ^                    |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {2:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name] [+]       }|
+                              |
+        ]]}
+      end)
+
+      it('of split windows with same buffer', function()
+        local win = meths.get_current_win()
+        local buf = meths.get_current_buf()
+
+        command('split')
+        meths.win_set_cursor(win, {8,0})
+        meths.win_set_cursor(0, {1,1})
+
+        screen:expect{grid=[[
+          a^aa                 |
+          bbb                 |
+          ccc                 |
+          ddd                 |
+          www                 |
+          {2:[No Name]           }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name]           }|
+                              |
+        ]]}
+        meths.buf_set_text(buf, 0,3, 1,0, {"X"})
+
+        screen:expect{grid=[[
+          a^aaXbbb             |
+          ccc                 |
+          ddd                 |
+          www                 |
+          xxx                 |
+          {2:[No Name] [+]       }|
+          www                 |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name] [+]       }|
+                              |
+        ]]}
+      end)
+    end)
   end)
 
   describe_lua_and_rpc('nvim_buf_get_text', function(api)