From b781e95a24bd2d201f4afbe4729635114893cdf5 Mon Sep 17 00:00:00 2001
From: Yanuo Ma <41042490+esmuellert@users.noreply.github.com>
Date: Thu, 27 Aug 2026 10:04:33 -0400
Subject: [PATCH] fix(scrollbind): window jumps back and forth with tall
 virt_lines block

Problem:
Scrolling a 'scrollbind' window whose peer has a virt_lines block
taller than the window makes the peer jump back and forth.

Solution:
After scrollup()/scrolldown(), sync w_scbind_pos to the actual
get_vtopline() when the target was not clamped to buffer bounds.

Ref: esmuellert/codediff.nvim#519
AI-assisted
---
 src/nvim/normal.c                      |  6 ++
 test/functional/ui/scrollbind_spec.lua | 84 ++++++++++++++++++++++++++
 2 files changed, 90 insertions(+)

diff --git a/src/nvim/normal.c b/src/nvim/normal.c
index b5cd75e663..7f877367f8 100644
--- a/src/nvim/normal.c
+++ b/src/nvim/normal.c
@@ -2185,6 +2185,12 @@ void check_scrollbind(linenr_T vtopline_diff, int leftcol_diff)
         } else {
           scrolldown(curwin, -y, false);
         }
+
+        // The scroll may not reach new_vtopline (tall virt_lines block).
+        // Sync w_scbind_pos to the actual position, else the miss repeats.
+        if (curwin->w_scbind_pos == new_vtopline) {
+          curwin->w_scbind_pos = get_vtopline(curwin);
+        }
       }
 
       redraw_later(curwin, UPD_VALID);
diff --git a/test/functional/ui/scrollbind_spec.lua b/test/functional/ui/scrollbind_spec.lua
index 54239fec1c..9081ee491e 100644
--- a/test/functional/ui/scrollbind_spec.lua
+++ b/test/functional/ui/scrollbind_spec.lua
@@ -439,4 +439,88 @@ describe('Scrollbind', function()
       ]],
     })
   end)
+
+  it('does not jump back and forth with a virt_lines block taller than the window', function()
+    n.exec_lua(function()
+      local lines = {} --- @type string[]
+      for i = 1, 20 do
+        lines[i] = tostring(i)
+      end
+
+      -- Bound window: one virt_lines block of 15 rows, window is 10 rows.
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+      vim.bo.buftype = 'nofile'
+      local virt_lines = {} --- @type table[]
+      for i = 1, 15 do
+        virt_lines[i] = { { 'v' .. i } }
+      end
+      vim.api.nvim_buf_set_extmark(0, vim.api.nvim_create_namespace('test'), 4, 0, {
+        virt_lines = virt_lines,
+      })
+      vim.wo.scrollbind = true
+
+      -- Scrolled window: plain buffer.
+      vim.cmd.vnew()
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+      vim.bo.buftype = 'nofile'
+      vim.wo.scrollbind = true
+    end)
+
+    n.feed('5<C-e>')
+
+    screen:expect({
+      grid = [[
+        ^6                   │v7                 |
+        7                   │v8                 |
+        8                   │v9                 |
+        9                   │v10                |
+        10                  │v11                |
+        11                  │v12                |
+        12                  │v13                |
+        13                  │v14                |
+        14                  │v15                |
+        15                  │6                  |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-e>')
+
+    screen:expect({
+      grid = [[
+        ^7                   │v8                 |
+        8                   │v9                 |
+        9                   │v10                |
+        10                  │v11                |
+        11                  │v12                |
+        12                  │v13                |
+        13                  │v14                |
+        14                  │v15                |
+        15                  │6                  |
+        16                  │7                  |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-e>')
+
+    screen:expect({
+      grid = [[
+        ^8                   │v9                 |
+        9                   │v10                |
+        10                  │v11                |
+        11                  │v12                |
+        12                  │v13                |
+        13                  │v14                |
+        14                  │v15                |
+        15                  │6                  |
+        16                  │7                  |
+        17                  │8                  |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+  end)
 end)