fix(scrollbind): properly take filler/virtual lines into account

Problem:

`'scrollbind'` does not work properly if the window being scrolled
automatically contains any filler/virtual lines (except for diff filler
lines).

This is because when the scrollbind check is done, the logic only
considers changes to topline which are represented as line numbers.

Solution:

Write the logic for determine the scroll amount to take into account
filler/virtual lines.

Fixes #29751

1 files changed, 442 insertions, 0 deletions

diff --git a/test/functional/ui/scrollbind_spec.lua b/test/functional/ui/scrollbind_spec.lua
new file mode 100644
index 0000000000..9e70b25efa
--- /dev/null
+++ b/test/functional/ui/scrollbind_spec.lua
@@ -0,0 +1,442 @@
+local t = require('test.testutil')
+local n = require('test.functional.testnvim')()
+local clear = n.clear
+local Screen = require('test.functional.ui.screen')
+
+before_each(clear)
+
+describe('Scrollbind', function()
+  local screen --- @type test.functional.ui.screen
+
+  before_each(function()
+    screen = Screen.new(40, 12)
+    screen:attach()
+  end)
+
+  it('works with one buffer with virtual lines', function()
+    n.exec_lua(function()
+      local lines = {} --- @type string[]
+
+      for i = 1, 20 do
+        lines[i] = tostring(i * 2 - 1)
+      end
+
+      local ns = vim.api.nvim_create_namespace('test')
+
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+      vim.bo.buftype = 'nofile'
+
+      for i in ipairs(lines) do
+        vim.api.nvim_buf_set_extmark(0, ns, i - 1, 0, {
+          virt_lines = { { { tostring(2 * i) .. ' v' } } },
+        })
+      end
+
+      vim.wo.scrollbind = true
+      vim.cmd.vsplit()
+      vim.wo.scrollbind = true
+    end)
+
+    n.feed('<C-d>')
+
+    t.eq(5, n.api.nvim_get_option_value('scroll', {}))
+
+    screen:expect({
+      grid = [[
+        6 v                 │6 v                |
+        7                   │7                  |
+        8 v                 │8 v                |
+        9                   │9                  |
+        10 v                │10 v               |
+        ^11                  │11                 |
+        12 v                │12 v               |
+        13                  │13                 |
+        14 v                │14 v               |
+        15                  │15                 |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-u>')
+
+    local line1_grid = [[
+      ^1                   │1                  |
+      2 v                 │2 v                |
+      3                   │3                  |
+      4 v                 │4 v                |
+      5                   │5                  |
+      6 v                 │6 v                |
+      7                   │7                  |
+      8 v                 │8 v                |
+      9                   │9                  |
+      10 v                │10 v               |
+      {3:[Scratch]            }{2:[Scratch]          }|
+                                              |
+    ]]
+
+    screen:expect({ grid = line1_grid })
+
+    n.api.nvim_set_option_value('scroll', 6, {})
+
+    n.feed('<C-d>')
+
+    screen:expect({
+      grid = [[
+        7                   │7                  |
+        8 v                 │8 v                |
+        9                   │9                  |
+        10 v                │10 v               |
+        11                  │11                 |
+        12 v                │12 v               |
+        ^13                  │13                 |
+        14 v                │14 v               |
+        15                  │15                 |
+        16 v                │16 v               |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-u>')
+
+    screen:expect({ grid = line1_grid })
+  end)
+
+  it('works with two buffers with virtual lines on one side', function()
+    n.exec_lua(function()
+      local lines = {} --- @type string[]
+
+      for i = 1, 20 do
+        lines[i] = tostring(i)
+      end
+
+      local ns = vim.api.nvim_create_namespace('test')
+
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+      vim.bo.buftype = 'nofile'
+
+      vim.wo.scrollbind = true
+      vim.cmd.vnew()
+
+      lines = {} --- @type string[]
+
+      for i = 1, 20 do
+        lines[i] = tostring(i + (i > 3 and 4 or 0))
+      end
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+      vim.bo.buftype = 'nofile'
+
+      vim.api.nvim_buf_set_extmark(0, ns, 2, 0, {
+        virt_lines = {
+          { { '4 v' } },
+          { { '5 v' } },
+          { { '6 v' } },
+          { { '7 v' } },
+        },
+      })
+
+      vim.wo.scrollbind = true
+    end)
+
+    n.feed('<C-d>')
+
+    t.eq(5, n.api.nvim_get_option_value('scroll', {}))
+
+    screen:expect({
+      grid = [[
+        6 v                 │6                  |
+        7 v                 │7                  |
+        8                   │8                  |
+        9                   │9                  |
+        ^10                  │10                 |
+        11                  │11                 |
+        12                  │12                 |
+        13                  │13                 |
+        14                  │14                 |
+        15                  │15                 |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-u>')
+
+    local line1_grid = [[
+      ^1                   │1                  |
+      2                   │2                  |
+      3                   │3                  |
+      4 v                 │4                  |
+      5 v                 │5                  |
+      6 v                 │6                  |
+      7 v                 │7                  |
+      8                   │8                  |
+      9                   │9                  |
+      10                  │10                 |
+      {3:[Scratch]            }{2:[Scratch]          }|
+                                              |
+    ]]
+
+    screen:expect({ grid = line1_grid })
+
+    n.api.nvim_set_option_value('scroll', 6, {})
+
+    n.feed('<C-d>')
+
+    screen:expect({
+      grid = [[
+        7 v                 │7                  |
+        8                   │8                  |
+        9                   │9                  |
+        10                  │10                 |
+        ^11                  │11                 |
+        12                  │12                 |
+        13                  │13                 |
+        14                  │14                 |
+        15                  │15                 |
+        16                  │16                 |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-u>')
+
+    screen:expect({ grid = line1_grid })
+
+    -- Note: not the same as n.feed('4<C-e>')
+    n.feed('<C-e>')
+    n.feed('<C-e>')
+    n.feed('<C-e>')
+    n.feed('<C-e>')
+
+    screen:expect({
+      grid = [[
+        5 v                 │5                  |
+        6 v                 │6                  |
+        7 v                 │7                  |
+        ^8                   │8                  |
+        9                   │9                  |
+        10                  │10                 |
+        11                  │11                 |
+        12                  │12                 |
+        13                  │13                 |
+        14                  │14                 |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-e>')
+
+    screen:expect({
+      grid = [[
+        6 v                 │6                  |
+        7 v                 │7                  |
+        ^8                   │8                  |
+        9                   │9                  |
+        10                  │10                 |
+        11                  │11                 |
+        12                  │12                 |
+        13                  │13                 |
+        14                  │14                 |
+        15                  │15                 |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-y>')
+    n.feed('<C-y>')
+
+    screen:expect({
+      grid = [[
+        4 v                 │4                  |
+        5 v                 │5                  |
+        6 v                 │6                  |
+        7 v                 │7                  |
+        ^8                   │8                  |
+        9                   │9                  |
+        10                  │10                 |
+        11                  │11                 |
+        12                  │12                 |
+        13                  │13                 |
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+  end)
+
+  it('works with buffers of different lengths', function()
+    n.exec_lua(function()
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, { '1', '2', '3' })
+      vim.bo.buftype = 'nofile'
+
+      vim.wo.scrollbind = true
+      vim.cmd.vnew()
+
+      local lines = {} --- @type string[]
+
+      for i = 1, 50 do
+        lines[i] = tostring(i)
+      end
+
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+      vim.bo.buftype = 'nofile'
+      vim.wo.scrollbind = true
+    end)
+
+    n.feed('10<C-e>')
+
+    screen:expect({
+      grid = [[
+        ^11                  │3                  |
+        12                  │{1:~                  }|
+        13                  │{1:~                  }|
+        14                  │{1:~                  }|
+        15                  │{1:~                  }|
+        16                  │{1:~                  }|
+        17                  │{1:~                  }|
+        18                  │{1:~                  }|
+        19                  │{1:~                  }|
+        20                  │{1:~                  }|
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-y>')
+
+    screen:expect({
+      grid = [[
+        10                  │3                  |
+        ^11                  │{1:~                  }|
+        12                  │{1:~                  }|
+        13                  │{1:~                  }|
+        14                  │{1:~                  }|
+        15                  │{1:~                  }|
+        16                  │{1:~                  }|
+        17                  │{1:~                  }|
+        18                  │{1:~                  }|
+        19                  │{1:~                  }|
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+  end)
+
+  it('works with buffers of different lengths and virtual lines', function()
+    n.exec_lua(function()
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, { '1', '5', '6' })
+
+      local ns = vim.api.nvim_create_namespace('test')
+      vim.api.nvim_buf_set_extmark(0, ns, 0, 0, {
+        virt_lines = {
+          { { '2 v' } },
+          { { '3 v' } },
+          { { '4 v' } },
+        },
+      })
+
+      vim.bo.buftype = 'nofile'
+
+      vim.wo.scrollbind = true
+      vim.cmd.vnew()
+
+      local lines = {} --- @type string[]
+
+      for i = 1, 50 do
+        lines[i] = tostring(i)
+      end
+
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+      vim.bo.buftype = 'nofile'
+      vim.wo.scrollbind = true
+    end)
+
+    n.feed('<C-e>')
+    n.feed('<C-e>')
+    screen:expect({
+      grid = [[
+        ^3                   │3 v                |
+        4                   │4 v                |
+        5                   │5                  |
+        6                   │6                  |
+        7                   │{1:~                  }|
+        8                   │{1:~                  }|
+        9                   │{1:~                  }|
+        10                  │{1:~                  }|
+        11                  │{1:~                  }|
+        12                  │{1:~                  }|
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('8<C-e>')
+
+    screen:expect({
+      grid = [[
+        ^11                  │6                  |
+        12                  │{1:~                  }|
+        13                  │{1:~                  }|
+        14                  │{1:~                  }|
+        15                  │{1:~                  }|
+        16                  │{1:~                  }|
+        17                  │{1:~                  }|
+        18                  │{1:~                  }|
+        19                  │{1:~                  }|
+        20                  │{1:~                  }|
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-y>')
+    n.feed('<C-y>')
+    n.feed('<C-y>')
+    n.feed('<C-y>')
+    n.feed('<C-y>')
+
+    t.eq(n.exec_lua [[return vim.fn.line('w0', 1001)]], 6)
+    t.eq(n.exec_lua [[return vim.fn.line('w0', 1000)]], 3)
+
+    screen:expect({
+      grid = [[
+        6                   │6                  |
+        7                   │{1:~                  }|
+        8                   │{1:~                  }|
+        9                   │{1:~                  }|
+        10                  │{1:~                  }|
+        ^11                  │{1:~                  }|
+        12                  │{1:~                  }|
+        13                  │{1:~                  }|
+        14                  │{1:~                  }|
+        15                  │{1:~                  }|
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+
+    n.feed('<C-y>')
+    n.feed('<C-y>')
+    n.feed('<C-y>')
+
+    screen:expect({
+      grid = [[
+        3                   │3 v                |
+        4                   │4 v                |
+        5                   │5                  |
+        6                   │6                  |
+        7                   │{1:~                  }|
+        8                   │{1:~                  }|
+        9                   │{1:~                  }|
+        10                  │{1:~                  }|
+        ^11                  │{1:~                  }|
+        12                  │{1:~                  }|
+        {3:[Scratch]            }{2:[Scratch]          }|
+                                                |
+      ]],
+    })
+  end)
+end)