fix: heading border with indentation

When heading borders are rendered on empty lines above the heading,
those lines also need to be indented to the next level.

Author: lukas-reineke

lua/render-markdown/handler/markdown.lua

@@ -69,9 +69,21 @@ function Handler:section(info)
         return
     end
 
+    local start_row = info.start_row
+    local heading_renderer = self.renderers.heading --[[@as render.md.render.Heading?]]
+    -- If heading borders are on, and we reuse an empty line in the buffer for the top border, that line needs to be
+    -- indented as well
+    if
+        self.config.heading.border
+        and heading_renderer
+        and heading_renderer:has_space_for_border('above', info, self.context.last_heading)
+    then
+        start_row = start_row - 1
+    end
+
     -- Each level stacks inline marks so we do not need to multiply spaces
     -- However skipping a level, i.e. 2 -> 5, will only add one level of spaces
-    for row = info.start_row, info.end_row - 1 do
+    for row = start_row, info.end_row - 1 do
         self.marks:add(false, row, 0, {
             priority = 0,
             virt_text = { { str.spaces(indent.per_level), 'Normal' } },

lua/render-markdown/render/heading.lua

@@ -72,6 +72,18 @@ function Render:render()
     self:conceal_underline()
 end
 
+---@param position 'above'|'below'
+---@param info render.md.NodeInfo
+---@param last_heading integer
+---@return boolean
+function Render:has_space_for_border(position, info, last_heading)
+    if position == 'above' then
+        return str.width(info:line('above')) == 0 and info.start_row - 1 ~= last_heading
+    else
+        return str.width(self.info:line('below')) == 0
+    end
+end
+
 ---@private
 ---@return integer
 function Render:icon()
@@ -174,7 +186,7 @@ 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 self:has_space_for_border('above', self.info, 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 +203,7 @@ 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 self:has_space_for_border('below', self.info, self.context.last_heading) then
         self.marks:add(true, self.info.end_row + 1, 0, {
             virt_text = line_below,
             virt_text_pos = 'overlay',