fix: heading borders with indentation (#164)

* fix: heading borders when there is no first heading

Don't assume that there is always a first level heading when counting
the indentation level.

* 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.

* fixup! fix: heading border with indentation

Author: lukas-reineke

lua/render-markdown/core/node_info.lua

@@ -58,7 +58,12 @@ function NodeInfo:level()
     local parent = self.node:parent()
     while parent ~= nil and parent:type() ~= 'document' do
         if parent:type() == 'section' then
-            level = level + 1
+            local child = parent:child(0)
+            child = child and child:child(0)
+            local is_first_heading = child and child:type() == 'atx_h1_marker'
+            if not is_first_heading then
+                level = level + 1
+            end
         end
         parent = parent:parent()
     end

lua/render-markdown/render/base.lua

@@ -41,13 +41,14 @@ end
 
 ---@protected
 ---@param line { [1]: string, [2]: string }[]
+---@param level integer?
 ---@return { [1]: string, [2]: string }[]
-function Base:indent_virt_line(line)
+function Base:indent_virt_line(line, level)
     local indent = self.config.indent
     if not indent.enabled then
         return line
     end
-    local level = self.info:level() - 1
+    level = level or self.info:level()
     if level <= 0 then
         return line
     end

lua/render-markdown/render/heading.lua

@@ -175,6 +175,9 @@ function Render:border(width)
         { 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.data.level > 1 then
+            line_above = self:indent_virt_line(line_above, 1)
+        end
         self.marks:add(true, self.info.start_row - 1, 0, {
             virt_text = line_above,
             virt_text_pos = 'overlay',