From 5f1f5a8c2a00145eb4bfbb5979efa14934ef90cf Mon Sep 17 00:00:00 2001
From: Volodymyr Chernetskyi <19735328+chernetskyi@users.noreply.github.com>
Date: Fri, 4 Sep 2026 18:25:14 +0200
Subject: [PATCH] fix(treesitter): conceal_lines is not applied to injected
 trees

Problem:
TSHighlighter._on_conceal_line() parses with the range { row, row }. A
Range2 has an exclusive end, so that is the empty range, and no injected
region ever intercepts it.

The root tree is parsed regardless, because its region is empty, so only
injections are affected: conceal_lines metadata coming from an injected
language's highlights query is dropped. on_range_impl() then records the
row in _conceal_checked, so the miss persists until the buffer changes.

For a markdown code block nested in a markdown code block, the inner
fence delimiters stay visible and nvim_win_text_height() reports 5 rows
where 3 are displayed.

Solution:
Pass the one-row range, as the on_range_impl() call below already does.

AI-assisted
---
 runtime/lua/vim/treesitter/highlighter.lua    |  2 +-
 test/functional/treesitter/highlight_spec.lua | 27 +++++++++++++++++++
 2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/runtime/lua/vim/treesitter/highlighter.lua b/runtime/lua/vim/treesitter/highlighter.lua
index b5ddd28aa2..f7e22eb5ff 100644
--- a/runtime/lua/vim/treesitter/highlighter.lua
+++ b/runtime/lua/vim/treesitter/highlighter.lua
@@ -549,7 +549,7 @@ function TSHighlighter._on_conceal_line(_, _, buf, row)
 
   -- Do not affect potentially populated highlight state.
   local highlight_states = self._highlight_states
-  self.tree:parse({ row, row })
+  self.tree:parse({ row, row + 1 })
   self:prepare_highlight_states(row, row)
   on_range_impl(self, buf, row, 0, row + 1, 0, false, true)
   self._highlight_states = highlight_states
diff --git a/test/functional/treesitter/highlight_spec.lua b/test/functional/treesitter/highlight_spec.lua
index a4ddf40e77..65dd4457f1 100644
--- a/test/functional/treesitter/highlight_spec.lua
+++ b/test/functional/treesitter/highlight_spec.lua
@@ -1553,3 +1553,30 @@ mispelledtwo]])
   local pos = api.nvim_win_get_cursor(0)
   eq(1, pos[1], 'Should have wrapped back to Line 1')
 end)
+
+it('conceals lines contributed by an injected tree', function()
+  clear()
+  command('set conceallevel=3')
+
+  -- Measured without an intervening redraw, so that the conceal_line callback
+  -- is the thing that has to parse the injection. The outer ~~~ fences come
+  -- from the root tree, the inner ``` fences from the markdown tree injected
+  -- into it; all four carry conceal_lines, leaving "filler", "print(1)" and
+  -- "tail" on screen.
+  eq(
+    3,
+    exec_lua(function()
+      vim.api.nvim_buf_set_lines(0, 0, -1, false, {
+        'filler',
+        '~~~markdown',
+        '```lua',
+        'print(1)',
+        '```',
+        '~~~',
+        'tail',
+      })
+      vim.treesitter.start(0, 'markdown')
+      return vim.api.nvim_win_text_height(0, {}).all
+    end)
+  )
+end)