feat(ui): render tool-call borders via statuscolumn (#215)

Render tool-call borders via `'statuscolumn'` so they repeat on
soft-wrap
and foldtext lines.

- Supersedes and closes #211.

## Design

- Reuses the existing `NS_TOOL_BLOCKS` range extmark. No cache.
- Per draw: `nvim_buf_get_extmarks(... overlap=true, limit=1)`. O(log
N).
- Write cost: zero.
- `inline` + `virt_text_repeat_linebreak` not supported
([neovim/neovim#35341](neovim/neovim#35341)).
- User `chat.win_opts.statuscolumn`/`winhighlight` wins via
`tbl_deep_extend`.
- Maps each themable column highlight group to `Normal` so the gutter
blends with the chat background instead of looking like a separate
column. `cursorline=false`.
- 1-cell glyph (`╭ / │ / ╰`).

## Performance

Real chat: 10371 lines, 272 blocks, 58 visible rows.

```
ns/call: 754
ms/redraw: 0.044
```

~1-2 ms/sec under heavy streaming.

## Alternatives rejected

- `inline` + repeat: upstream wontfix
([neovim/neovim#35341](neovim/neovim#35341)).
- `overlay` + repeat: pollutes copy/paste.
- Signs: no soft-wrap repeat; shared column.
- `'showbreak'`: window-wide.
- Lua cache: O(N²) per session.

Author: carlos-algms

lua/agentic/acp/AGENTS.md

@@ -106,7 +106,8 @@ Provider sends "tool_call"
   -> MessageWriter:write_tool_call_block(block)
      1. Renders header + body/diff lines to buffer
      2. Creates range extmark (NS_TOOL_BLOCKS) as position anchor
-     3. Creates decoration extmarks (borders, status icon)
+     3. Statuscolumn reads the range extmark for borders; status footer
+        extmark renders the status icon
      4. Stores block in tool_call_blocks[id]
 ```
 
@@ -122,7 +123,7 @@ Provider sends "tool_call_update"
      2. Deep-merges via tbl_deep_extend("force", tracker, partial)
      3. Appends body (if both old and new exist and differ)
      4. Locates block position via range extmark
-     5. Diff already rendered: refresh decorations + status only
+     5. Diff already rendered: refresh header + status only
         (content frozen to prevent flicker)
      6. Diff is NEW: replace buffer lines, re-render everything
 ```
@@ -140,8 +141,7 @@ Same as Phase 2, but status = "completed" | "failed"
 - **Updates are partial:** Only send what changed. MessageWriter merges onto the
   existing tracker via `tbl_deep_extend`.
 - **Diffs are immutable after first render:** Once a diff is written to the
-  buffer, content is frozen. Only status/decorations refresh on subsequent
-  updates.
+  buffer, content is frozen. Only header/status refresh on subsequent updates.
 - **Body accumulates:** Multiple updates with different body content get
   concatenated with `---` dividers, not replaced.
 - **Extmarks as position anchors:** Range extmark in `NS_TOOL_BLOCKS`

lua/agentic/ui/AGENTS.md

@@ -29,7 +29,7 @@ SessionManager (per tab)
         ├── ToolCallFold        manual folds, anchor pads
         ├── ToolCallDiff        diff extraction + minimization
         ├── DiffHighlighter     line/word hl on chat buffer
-        ├── ExtmarkBlock        ╭ │ ╰ fence glyphs
+        ├── ToolBlockBorder     ╭ │ ╰ fence glyphs via chat statuscolumn
         └── PermissionManager   queues + reanchors permission prompts
 ```
 
@@ -183,9 +183,9 @@ row N    "" trailing      footer, status virt_text
 
 ```text
 tracker missing       ─▶ debug-log, return
-already_has_diff      ─▶ refresh header + status + decorations only
+already_has_diff      ─▶ refresh header + status only
 otherwise             ─▶ rewrite body between anchors,
-                         re-apply highlights + decorations,
+                         re-apply highlights + range anchor,
                          create fold if interior crosses threshold
 ```
 

lua/agentic/ui/message_writer.lua

@@ -3,14 +3,12 @@ local BufHelpers = require("agentic.utils.buf_helpers")
 local Config = require("agentic.config")
 local DiffHighlighter = require("agentic.utils.diff_highlighter")
 local DiffPreview = require("agentic.ui.diff_preview")
-local ExtmarkBlock = require("agentic.utils.extmark_block")
 local Fold = require("agentic.ui.tool_call_fold")
 local JsonFormat = require("agentic.utils.json_format")
 local Logger = require("agentic.utils.logger")
 local Theme = require("agentic.theme")
 
 local NS_TOOL_BLOCKS = vim.api.nvim_create_namespace("agentic_tool_blocks")
-local NS_DECORATIONS = vim.api.nvim_create_namespace("agentic_tool_decorations")
 local NS_PERMISSION_BUTTONS =
     vim.api.nvim_create_namespace("agentic_permission_buttons")
 local NS_DIFF_HIGHLIGHTS =
@@ -35,7 +33,6 @@ local NS_THINKING = vim.api.nvim_create_namespace("agentic_thinking")
 --- @field argument? string
 --- @field file_path? string
 --- @field extmark_id? integer Range extmark spanning the block
---- @field decoration_extmark_ids? integer[] IDs of decoration extmarks from ExtmarkBlock
 --- @field status? agentic.acp.ToolCallStatus
 --- @field body? string[]
 --- @field diff? agentic.ui.MessageWriter.ToolCallDiff
@@ -425,15 +422,6 @@ function MessageWriter:write_tool_call_block(tool_call_block)
             highlight_ranges
         )
 
-        tool_call_block.decoration_extmark_ids =
-            ExtmarkBlock.render_block(bufnr, NS_DECORATIONS, {
-                header_line = start_row,
-                body_start = start_row + 1,
-                body_end = end_row - 1,
-                footer_line = end_row,
-                hl_group = Theme.HL_GROUPS.CODE_BLOCK_FENCE,
-            })
-
         tool_call_block.extmark_id =
             vim.api.nvim_buf_set_extmark(bufnr, NS_TOOL_BLOCKS, start_row, 0, {
                 id = tool_call_block.extmark_id,
@@ -544,10 +532,6 @@ function MessageWriter:update_tool_call_block(tool_call_block)
                 return false
             end
 
-            self:_clear_decoration_extmarks(tracker.decoration_extmark_ids)
-            tracker.decoration_extmark_ids =
-                self:_render_decorations(start_row, old_end_row)
-
             self:_clear_status_namespace(start_row, old_end_row)
             self:_apply_status_highlights_if_present(
                 start_row,
@@ -558,7 +542,6 @@ function MessageWriter:update_tool_call_block(tool_call_block)
             return false
         end
 
-        self:_clear_decoration_extmarks(tracker.decoration_extmark_ids)
         self:_clear_status_namespace(start_row, old_end_row)
 
         local new_lines, highlight_ranges = self:_prepare_block_lines(tracker)
@@ -596,9 +579,6 @@ function MessageWriter:update_tool_call_block(tool_call_block)
             right_gravity = false,
         })
 
-        tracker.decoration_extmark_ids =
-            self:_render_decorations(start_row, new_end_row)
-
         self:_apply_status_highlights_if_present(
             start_row,
             new_end_row,
@@ -1146,30 +1126,6 @@ function MessageWriter:_set_thinking_extmark(start_line, end_line, id)
     )
 end
 
---- @param ids integer[]|nil
-function MessageWriter:_clear_decoration_extmarks(ids)
-    if not ids then
-        return
-    end
-
-    for _, id in ipairs(ids) do
-        pcall(vim.api.nvim_buf_del_extmark, self.bufnr, NS_DECORATIONS, id)
-    end
-end
-
---- @param start_row integer
---- @param end_row integer
---- @return integer[] decoration_extmark_ids
-function MessageWriter:_render_decorations(start_row, end_row)
-    return ExtmarkBlock.render_block(self.bufnr, NS_DECORATIONS, {
-        header_line = start_row,
-        body_start = start_row + 1,
-        body_end = end_row - 1,
-        footer_line = end_row,
-        hl_group = Theme.HL_GROUPS.CODE_BLOCK_FENCE,
-    })
-end
-
 --- @param start_row integer
 --- @param end_row integer
 function MessageWriter:_clear_status_namespace(start_row, end_row)

lua/agentic/ui/tool_block_border.lua

@@ -0,0 +1,145 @@
+local Theme = require("agentic.theme")
+
+local NS_TOOL_BLOCKS = vim.api.nvim_create_namespace("agentic_tool_blocks")
+
+local GLYPHS = {
+    TOP_LEFT = "╭",
+    BOTTOM_LEFT = "╰",
+    HORIZONTAL = "─",
+    VERTICAL = "│",
+}
+
+local BLANK = " "
+
+local STATUSCOLUMN_EXPR =
+    "%!v:lua.require'agentic.ui.tool_block_border'.statuscolumn()"
+
+--- @class agentic.ui.ToolBlockBorder.Range
+--- @field start_row integer
+--- @field end_row integer
+
+--- @class agentic.ui.ToolBlockBorder
+local ToolBlockBorder = {
+    NS_TOOL_BLOCKS = NS_TOOL_BLOCKS,
+    STATUSCOLUMN_EXPR = STATUSCOLUMN_EXPR,
+}
+
+--- @return integer|nil bufnr
+local function statuscolumn_bufnr()
+    local winid = vim.g.statusline_winid
+    if type(winid) == "number" and vim.api.nvim_win_is_valid(winid) then
+        return vim.api.nvim_win_get_buf(winid)
+    end
+    local current = vim.api.nvim_get_current_buf()
+    if vim.api.nvim_buf_is_valid(current) then
+        return current
+    end
+    return nil
+end
+
+--- @param text string
+--- @return string text
+local function with_highlight(text)
+    return "%#" .. Theme.HL_GROUPS.CODE_BLOCK_FENCE .. "#" .. text .. "%*"
+end
+
+--- @param bufnr integer
+--- @param row integer
+--- @return agentic.ui.ToolBlockBorder.Range|nil range
+function ToolBlockBorder.block_range_at_row(bufnr, row)
+    if row < 0 or not vim.api.nvim_buf_is_valid(bufnr) then
+        return nil
+    end
+
+    local ok, extmarks = pcall(
+        vim.api.nvim_buf_get_extmarks,
+        bufnr,
+        NS_TOOL_BLOCKS,
+        {
+            row,
+            0,
+        },
+        { row, -1 },
+        {
+            details = true,
+            limit = 1,
+            overlap = true,
+        }
+    )
+
+    if not ok or not extmarks or not extmarks[1] then
+        return nil
+    end
+
+    local extmark = extmarks[1]
+    local start_row = extmark[2]
+    local details = extmark[4] or {}
+    local end_row = details.end_row
+
+    if not end_row or row < start_row or row > end_row then
+        return nil
+    end
+
+    --- @type agentic.ui.ToolBlockBorder.Range
+    local range = {
+        start_row = start_row,
+        end_row = end_row,
+    }
+    return range
+end
+
+--- @param bufnr integer
+--- @param row integer
+--- @param virtnum integer
+--- @return string glyph
+function ToolBlockBorder.glyph_for_line(bufnr, row, virtnum)
+    if virtnum < 0 then
+        return BLANK
+    end
+
+    local range = ToolBlockBorder.block_range_at_row(bufnr, row)
+    if not range then
+        return BLANK
+    end
+
+    if virtnum > 0 then
+        return GLYPHS.VERTICAL
+    end
+
+    if row == range.start_row then
+        return GLYPHS.TOP_LEFT
+    end
+
+    if row == range.end_row then
+        return GLYPHS.BOTTOM_LEFT
+    end
+
+    return GLYPHS.VERTICAL
+end
+
+--- @return string text
+local function render_statuscolumn()
+    local bufnr = statuscolumn_bufnr()
+    if not bufnr then
+        return BLANK
+    end
+
+    local glyph =
+        ToolBlockBorder.glyph_for_line(bufnr, vim.v.lnum - 1, vim.v.virtnum)
+    if glyph == BLANK then
+        return BLANK
+    end
+
+    return with_highlight(glyph)
+end
+
+--- @return string text
+function ToolBlockBorder.statuscolumn()
+    local ok, text = pcall(render_statuscolumn)
+    if ok then
+        return text
+    end
+    return BLANK
+end
+
+return ToolBlockBorder

lua/agentic/ui/tool_block_border.test.lua

@@ -0,0 +1,81 @@
+local assert = require("tests.helpers.assert")
+local ToolBlockBorder = require("agentic.ui.tool_block_border")
+
+describe("agentic.ui.ToolBlockBorder", function()
+    local bufnr
+
+    before_each(function()
+        bufnr = vim.api.nvim_create_buf(false, true)
+        vim.bo[bufnr].buftype = "nofile"
+        vim.bo[bufnr].swapfile = false
+        vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, {
+            "before",
+            "header",
+            "",
+            "body",
+            "",
+            "",
+            "between",
+            "header two",
+            "body two",
+            "",
+        })
+    end)
+
+    after_each(function()
+        if bufnr and vim.api.nvim_buf_is_valid(bufnr) then
+            vim.api.nvim_buf_delete(bufnr, { force = true })
+        end
+    end)
+
+    --- @param start_row integer
+    --- @param end_row integer
+    local function add_block(start_row, end_row)
+        vim.api.nvim_buf_set_extmark(
+            bufnr,
+            ToolBlockBorder.NS_TOOL_BLOCKS,
+            start_row,
+            0,
+            {
+                end_row = end_row,
+                right_gravity = false,
+            }
+        )
+    end
+
+    it(
+        "returns corner and body glyphs from the anchored block range",
+        function()
+            add_block(1, 5)
+
+            assert.equal("╭", ToolBlockBorder.glyph_for_line(bufnr, 1, 0))
+            assert.equal("│", ToolBlockBorder.glyph_for_line(bufnr, 2, 0))
+            assert.equal("│", ToolBlockBorder.glyph_for_line(bufnr, 3, 0))
+            assert.equal("╰", ToolBlockBorder.glyph_for_line(bufnr, 5, 0))
+        end
+    )
+
+    it("uses body glyphs for wrapped continuations", function()
+        add_block(1, 5)
+
+        assert.equal("│", ToolBlockBorder.glyph_for_line(bufnr, 1, 1))
+        assert.equal("│", ToolBlockBorder.glyph_for_line(bufnr, 3, 2))
+        assert.equal("│", ToolBlockBorder.glyph_for_line(bufnr, 5, 1))
+    end)
+
+    it("uses a body glyph for folded body screen lines", function()
+        add_block(1, 5)
+
+        assert.equal("│", ToolBlockBorder.glyph_for_line(bufnr, 2, 0))
+    end)
+
+    it("does not bleed borders into lines outside or between blocks", function()
+        add_block(1, 5)
+        add_block(7, 9)
+
+        assert.equal(" ", ToolBlockBorder.glyph_for_line(bufnr, 0, 0))
+        assert.equal(" ", ToolBlockBorder.glyph_for_line(bufnr, 6, 0))
+        assert.equal("╭", ToolBlockBorder.glyph_for_line(bufnr, 7, 0))
+        assert.equal("╰", ToolBlockBorder.glyph_for_line(bufnr, 9, 0))
+    end)
+end)