fix(api): better topline adjustments in nvim_buf_set_lines

Some more reasonable defaults for topline:
  - if topline was replaced with another line, that now becomes topline
  - if line was inserted just before topline, display it. This is more
    similar to the previous API behavior.

3 files changed, 144 insertions, 5 deletions

diff --git a/src/nvim/api/buffer.c b/src/nvim/api/buffer.c
index d36f0dd050..b8cb09ceb3 100644
--- a/src/nvim/api/buffer.c
+++ b/src/nvim/api/buffer.c
@@ -464,7 +464,8 @@ void nvim_buf_set_lines(uint64_t channel_id, Buffer buffer, Integer start, Integ
 
   // Adjust marks. Invalidate any which lie in the
   // changed range, and move any in the remainder of the buffer.
-  mark_adjust_buf(buf, (linenr_T)start, (linenr_T)(end - 1), MAXLNUM, (linenr_T)extra,
+  linenr_T adjust = end > start ? MAXLNUM : 0;
+  mark_adjust_buf(buf, (linenr_T)start, (linenr_T)(end - 1), adjust, (linenr_T)extra,
                   true, true, kExtmarkNOOP);
 
   extmark_splice(buf, (int)start - 1, 0, (int)(end - start), 0,
@@ -711,7 +712,8 @@ void nvim_buf_set_text(uint64_t channel_id, Buffer buffer, Integer start_row, In
   // Adjust marks. Invalidate any which lie in the
   // 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,
+  linenr_T adjust = end_row >= start_row ? MAXLNUM : 0;
+  mark_adjust_buf(buf, (linenr_T)start_row, (linenr_T)end_row, adjust, (linenr_T)extra,
                   true, true, kExtmarkNOOP);
 
   extmark_splice(buf, (int)start_row - 1, (colnr_T)start_col,
diff --git a/src/nvim/mark.c b/src/nvim/mark.c
index 584a6c5827..39fc623676 100644
--- a/src/nvim/mark.c
+++ b/src/nvim/mark.c
@@ -1249,7 +1249,8 @@ void mark_adjust_buf(buf_T *buf, linenr_T line1, linenr_T line2, linenr_T amount
             if (line1 <= 1) {
               win->w_topline = 1;
             } else {
-              win->w_topline = line1 - 1;
+              // api: if the deleted region was replaced with new contents, display that
+              win->w_topline = (by_api && amount_after > line1 - line2 - 1) ? line1 : line1 - 1;
             }
           } else if (win->w_topline > line1) {
             // keep topline on the same line, unless inserting just
@@ -1257,7 +1258,9 @@ void mark_adjust_buf(buf_T *buf, linenr_T line1, linenr_T line2, linenr_T amount
             win->w_topline += amount;
           }
           win->w_topfill = 0;
-        } else if (amount_after && win->w_topline > line2) {
+          // api: display new line if inserted right at topline
+          // TODO(bfredl): maybe always?
+        } else if (amount_after && win->w_topline > line2 + (by_api ? 1 : 0)) {
           win->w_topline += amount_after;
           win->w_topfill = 0;
         }
diff --git a/test/functional/api/buffer_spec.lua b/test/functional/api/buffer_spec.lua
index 73a8749682..aea1f359e6 100644
--- a/test/functional/api/buffer_spec.lua
+++ b/test/functional/api/buffer_spec.lua
@@ -182,8 +182,8 @@ describe('api/buf', function()
           {2:[No Name]           }|
                               |
         ]]}
-        meths.buf_set_lines(buf, 0, 2, true, {"aaabbb"})
 
+        meths.buf_set_lines(buf, 0, 2, true, {"aaabbb"})
         screen:expect{grid=[[
                               |
           {1:~                   }|
@@ -198,6 +198,72 @@ describe('api/buf', function()
           {2:[No Name] [+]       }|
                               |
         ]]}
+
+        -- replacing topline keeps it the topline
+        meths.buf_set_lines(buf, 3, 4, true, {"wwweeee"})
+        screen:expect{grid=[[
+                              |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {3:[No Name]           }|
+          wwweeee             |
+          xxx                 |
+          yyy                 |
+          ^zzz                 |
+          {2:[No Name] [+]       }|
+                              |
+        ]]}
+
+        -- inserting just before topline does not scroll up if cursor would be moved
+        meths.buf_set_lines(buf, 3, 3, true, {"mmm"})
+        screen:expect{grid=[[
+                              |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {3:[No Name]           }|
+          wwweeee             |
+          xxx                 |
+          yyy                 |
+          ^zzz                 |
+          {2:[No Name] [+]       }|
+                              |
+        ]], unchanged=true}
+
+        meths.win_set_cursor(0, {7, 0})
+        screen:expect{grid=[[
+                              |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {3:[No Name]           }|
+          wwweeee             |
+          xxx                 |
+          ^yyy                 |
+          zzz                 |
+          {2:[No Name] [+]       }|
+                              |
+        ]]}
+
+        meths.buf_set_lines(buf, 4, 4, true, {"mmmeeeee"})
+        screen:expect{grid=[[
+                              |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {3:[No Name]           }|
+          mmmeeeee            |
+          wwweeee             |
+          xxx                 |
+          ^yyy                 |
+          {2:[No Name] [+]       }|
+                              |
+        ]]}
       end)
 
       it('of non-current window', function()
@@ -237,6 +303,40 @@ describe('api/buf', function()
           {3:[No Name] [+]       }|
                               |
         ]]}
+
+        -- replacing topline keeps it the topline
+        meths.buf_set_lines(buf, 3, 4, true, {"wwweeee"})
+        screen:expect{grid=[[
+          ^                    |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {2:[No Name]           }|
+          wwweeee             |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name] [+]       }|
+                              |
+        ]]}
+
+        -- inserting just before topline scrolls up
+        meths.buf_set_lines(buf, 3, 3, true, {"mmm"})
+        screen:expect{grid=[[
+          ^                    |
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {1:~                   }|
+          {2:[No Name]           }|
+          mmm                 |
+          wwweeee             |
+          xxx                 |
+          yyy                 |
+          {3:[No Name] [+]       }|
+                              |
+        ]]}
       end)
 
       it('of split windows with same buffer', function()
@@ -277,6 +377,40 @@ describe('api/buf', function()
           {3:[No Name] [+]       }|
                               |
         ]]}
+
+        -- replacing topline keeps it the topline
+        meths.buf_set_lines(buf, 3, 4, true, {"wwweeee"})
+        screen:expect{grid=[[
+          ^aaabbb              |
+          ccc                 |
+          ddd                 |
+          wwweeee             |
+          xxx                 |
+          {2:[No Name] [+]       }|
+          wwweeee             |
+          xxx                 |
+          yyy                 |
+          zzz                 |
+          {3:[No Name] [+]       }|
+                              |
+        ]]}
+
+        -- inserting just before topline scrolls up
+        meths.buf_set_lines(buf, 3, 3, true, {"mmm"})
+        screen:expect{grid=[[
+          ^aaabbb              |
+          ccc                 |
+          ddd                 |
+          mmm                 |
+          wwweeee             |
+          {2:[No Name] [+]       }|
+          mmm                 |
+          wwweeee             |
+          xxx                 |
+          yyy                 |
+          {3:[No Name] [+]       }|
+                              |
+        ]]}
       end)
     end)