fix: symbol hierarchy for markdown backend (#454)

Author: stevearc

lua/aerial/backends/markdown.lua

@@ -23,7 +23,10 @@ M.fetch_symbols_sync = function(bufnr)
     local idx, len = string.find(line, "^#+ ")
     if idx == 1 and not inside_code_block then
       local level = len - 2
-      local parent = stack[math.min(level, #stack)]
+      while #stack > 0 and stack[#stack].level >= level do
+        table.remove(stack, #stack)
+      end
+      local parent = stack[#stack]
       local item = {
         kind = "Interface",
         name = string.sub(line, len + 1),
@@ -49,9 +52,6 @@ M.fetch_symbols_sync = function(bufnr)
           table.insert(items, item)
         end
       end
-      while #stack > level and #stack > 0 do
-        table.remove(stack, #stack)
-      end
       table.insert(stack, item)
     elseif string.find(line, "```") == 1 then
       inside_code_block = not inside_code_block

lua/aerial/data.lua

@@ -109,7 +109,7 @@ end
 
 ---@param item aerial.Symbol
 ---@return nil|aerial.Symbol
-function BufData:_next_non_collapsed(item)
+local function _next_non_collapsed(item)
   while item do
     if item.next_sibling then
       return item.next_sibling
@@ -133,7 +133,7 @@ function BufData:iter(opts)
     ---@type nil|aerial.Symbol
     local item = self.flat_items[i]
     if opts.skip_hidden and item and self:is_collapsed(item.parent) then
-      item = self:_next_non_collapsed(item.parent)
+      item = _next_non_collapsed(item.parent)
     end
     if item then
       return item.idx, item, j