fix: indenting heading borders with single empty line between

## Details

When looking at headings separated by a single blank line and no other
content the alignement would of heading borders was not calculated
properly with indent and heading borders enabled. For example with the
following text:

```text
# Heading 1

## Heading 2

# Heading 1
```

We would end up overindent the bottom border of the first heading and
under indenting the bottom row of the seconding heading.

Both of these were caused by how section ownership was changed in a
recent change. In terms ownership the second heading owns the line below
the first heading and the third heading owns the line below the second
heading. However when actually adding a border we assume top down
ownership. By adding a line owned by a different section we end up with
the misalignment.

To fix this only extend ownership of a section upwards if there is more
than a single blank line. If it is just the single line than do not
extend the section. Very similar logic applies for contracting upward.
Only do this if the section does not rely on that blank line to exist.

Author: MeanderingProgrammer

lua/render-markdown/core/node_info.lua

@@ -127,18 +127,19 @@ function NodeInfo:for_each_child(callback)
     end
 end
 
----@param position 'above'|'below'|'first'|'last'
+---@param position 'above'|'first'|'below'|'last'
+---@param by integer
 ---@return string?
-function NodeInfo:line(position)
-    local row = nil
+function NodeInfo:line(position, by)
+    local one_line, row = self.start_row == self.end_row, nil
     if position == 'above' then
-        row = self.start_row - 1
-    elseif position == 'below' then
-        row = self.end_row + 1
+        row = self.start_row - by
     elseif position == 'first' then
-        row = self.start_row
+        row = self.start_row + by
+    elseif position == 'below' then
+        row = self.end_row - (one_line and 0 or 1) + by
     elseif position == 'last' then
-        row = self.end_row - 1
+        row = self.end_row - (one_line and 0 or 1) - by
     end
     return row ~= nil and vim.api.nvim_buf_get_lines(self.buf, row, row + 1, false)[1] or nil
 end

lua/render-markdown/handler/markdown_inline.lua

@@ -67,7 +67,7 @@ function Handler:shortcut(info)
         return
     end
 
-    local line = info:line('first')
+    local line = info:line('first', 0)
     if line ~= nil and line:find('[' .. info.text .. ']', 1, true) ~= nil then
         self:wiki_link(info)
         return

lua/render-markdown/health.lua

@@ -5,7 +5,7 @@ local M = {}
 
 ---@private
 ---@type string
-M.version = '6.3.9'
+M.version = '6.3.10'
 
 function M.check()
     vim.health.start('render-markdown.nvim [version]')

lua/render-markdown/render/heading.lua

@@ -174,7 +174,7 @@ function Render:border(width)
         { self.heading.above:rep(prefix), self.data.foreground },
         { self.heading.above:rep(width - self.heading.left_pad - prefix), background },
     }
-    if str.width(self.info:line('above')) == 0 and self.info.start_row - 1 ~= self.context.last_heading then
+    if str.width(self.info:line('above', 1)) == 0 and self.info.start_row - 1 ~= self.context.last_heading then
         self.marks:add(true, self.info.start_row - 1, 0, {
             virt_text = line_above,
             virt_text_pos = 'overlay',
@@ -191,7 +191,7 @@ function Render:border(width)
         { self.heading.below:rep(prefix), self.data.foreground },
         { self.heading.below:rep(width - self.heading.left_pad - prefix), background },
     }
-    if str.width(self.info:line('below')) == 0 then
+    if str.width(self.info:line('below', 1)) == 0 then
         self.marks:add(true, self.info.end_row + 1, 0, {
             virt_text = line_below,
             virt_text_pos = 'overlay',

lua/render-markdown/render/section.lua

@@ -36,12 +36,20 @@ function Render:setup()
 end
 
 function Render:render()
-    -- Do include empty line in previous section
-    local start_offset = str.width(self.info:line('above')) == 0 and 1 or 0
+    -- Include last empty line in previous section
+    -- Exclude if it is the only empty line in that section
+    local above, two_above = self.info:line('above', 1), self.info:line('above', 2)
+    local above_is_empty = str.width(above) == 0
+    local two_above_is_section = two_above ~= nil and vim.startswith(two_above, '#')
+    local start_offset = (above_is_empty and not two_above_is_section) and 1 or 0
     local start_row = math.max(self.info.start_row - start_offset, 0)
 
-    -- Do not include empty line at the end of current section
-    local end_offset = str.width(self.info:line('last')) == 0 and 1 or 0
+    -- Exclude last empty line in current section
+    -- Include if it is the only empty line of the last subsection
+    local last, second_last = self.info:line('last', 0), self.info:line('last', 1)
+    local last_is_empty = str.width(last) == 0
+    local second_last_is_section = second_last ~= nil and vim.startswith(second_last, '#')
+    local end_offset = (last_is_empty and not second_last_is_section) and 1 or 0
     local end_row = self.info.end_row - 1 - end_offset
 
     -- Each level stacks inline marks so we only need to multiply based on any skipped levels