fix: use nil if bg is missing in bg_as_fg

## Details

This should improve behavior with transparent color schemes based on
past issues / discussions. Unsure if it handles all situations but
should generally improve default behaviors. If the `bg` value we are
attempting to pull from a highlight group is missing the generated
highlight is labelled as not `visible`, in which case we return `nil`
rather than the actual highlight group. Update the logic to handle `nil`
values, only needed to be done when generating fenced code block border.

Author: MeanderingProgrammer

CHANGELOG.md

@@ -10,6 +10,7 @@
 - left padding for checkboxes [da260dd](https://github.com/MeanderingProgrammer/render-markdown.nvim/commit/da260dd4979d130f0dfc63e824c9cb2433cfded2)
 - dash width function [#537](https://github.com/MeanderingProgrammer/render-markdown.nvim/issues/537)
   [691651d](https://github.com/MeanderingProgrammer/render-markdown.nvim/commit/691651de4e02cbea9ff50c62d7d3a679abc95564)
+- implement preview split [72f658d](https://github.com/MeanderingProgrammer/render-markdown.nvim/commit/72f658d7c506fff6a10bee4e7fa2af081f91489b)
 
 ### Bug Fixes
 

doc/render-markdown.txt

@@ -1,4 +1,4 @@
-*render-markdown.txt*       For NVIM v0.11.4      Last change: 2025 October 16
+*render-markdown.txt*       For NVIM v0.11.4      Last change: 2025 October 24
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

lua/render-markdown/core/colors.lua

@@ -69,7 +69,7 @@ M.colors = {
 M.cache = {}
 ---@type table<string, { fg: string, bg: string }>
 M.cache.combine = {}
----@type table<string, { hl: string }>
+---@type table<string, { hl: string, visible: boolean }>
 M.cache.bg_as_fg = {}
 
 ---called from plugin directory
@@ -118,7 +118,7 @@ end
 
 ---@param highlight string
 ---@param force? boolean
----@return string
+---@return string?
 function M.bg_as_fg(highlight, force)
     local name = ('%s_%s_bg_as_fg'):format(M.prefix, highlight)
     if not M.cache.bg_as_fg[name] or force then
@@ -127,9 +127,9 @@ function M.bg_as_fg(highlight, force)
             fg = hl.bg,
             ctermfg = hl.ctermbg,
         })
-        M.cache.bg_as_fg[name] = { hl = highlight }
+        M.cache.bg_as_fg[name] = { hl = highlight, visible = hl.bg ~= nil }
     end
-    return name
+    return M.cache.bg_as_fg[name].visible and name or nil
 end
 
 ---@private

lua/render-markdown/health.lua

@@ -6,7 +6,7 @@ local state = require('render-markdown.state')
 local M = {}
 
 ---@private
-M.version = '8.9.9'
+M.version = '8.9.10'
 
 function M.check()
     M.start('versions')

lua/render-markdown/render/markdown/code.lua

@@ -175,23 +175,28 @@ end
 ---@param thin string
 function Render:border(node, thin)
     local kind = self.config.border
-    local highlight = self.config.highlight_border
+    if kind == 'none' or not node then
+        return
+    end
     if kind == 'hide' then
         self.marks:over(self.config, true, node, { conceal_lines = '' })
-    elseif kind == 'none' or not node or not highlight then
-        -- do nothing
-    else
-        local icon = kind == 'thin' and thin or ' '
-        if icon ~= ' ' then
-            highlight = colors.bg_as_fg(highlight)
-        end
-        local block = self.config.width == 'block'
-        local width = block and self.data.body - node.start_col or vim.o.columns
-        self.marks:start(self.config, 'code_border', node, {
-            virt_text = { { icon:rep(width), highlight } },
-            virt_text_pos = 'overlay',
-        })
+        return
     end
+    local highlight = self.config.highlight_border or nil
+    if not highlight then
+        return
+    end
+    local icon = kind == 'thin' and thin or ' '
+    highlight = icon == ' ' and highlight or colors.bg_as_fg(highlight)
+    if not highlight then
+        return
+    end
+    local block = self.config.width == 'block'
+    local width = block and self.data.body - node.start_col or vim.o.columns
+    self.marks:start(self.config, 'code_border', node, {
+        virt_text = { { icon:rep(width), highlight } },
+        virt_text_pos = 'overlay',
+    })
 end
 
 ---@private