refactor(renderer): address review feedback - extract folds_equal, simplify update_part_folds loop

- Extract folds_equal() helper function with nil-guard
- Simplify update_part_folds loop by assigning ctx.part_folds first
- Add 3 test cases for update_part_folds: absolute range computation,
  unchanged fold short-circuit, and multi-part fold merging

Author: CnsMaple

lua/opencode/ui/renderer/buffer.lua

@@ -657,6 +657,15 @@ function M.set_all_folds()
   output_window.set_folds(all_folds)
 end
 
+local function folds_equal(a, b)
+  if not a or not b then return false end
+  if #a ~= #b then return false end
+  for i = 1, #a do
+    if a[i].from ~= b[i].from or a[i].to ~= b[i].to then return false end
+  end
+  return true
+end
+
 ---Update folds for a single part during streaming, avoiding a full rebuild.
 ---@param part_id string
 function M.update_part_folds(part_id)
@@ -679,41 +688,17 @@ function M.update_part_folds(part_id)
     })
   end
 
-  local old_folds = ctx.part_folds[part_id]
-  if old_folds and #old_folds == #new_folds then
-    local same = true
-    for i = 1, #new_folds do
-      if new_folds[i].from ~= old_folds[i].from or new_folds[i].to ~= old_folds[i].to then
-        same = false
-        break
-      end
-    end
-    if same then
-      return
-    end
+  if folds_equal(ctx.part_folds[part_id], new_folds) then
+    return
   end
 
+  ctx.part_folds[part_id] = new_folds
   local new_global = {}
-  local found = false
-  for pid, pf in pairs(ctx.part_folds) do
-    if pid == part_id then
-      found = true
-      for _, nf in ipairs(new_folds) do
-        table.insert(new_global, nf)
-      end
-    else
-      for _, of in ipairs(pf) do
-        table.insert(new_global, of)
-      end
+  for _, pf in pairs(ctx.part_folds) do
+    for _, f in ipairs(pf) do
+      table.insert(new_global, f)
     end
   end
-  if not found then
-    for _, nf in ipairs(new_folds) do
-      table.insert(new_global, nf)
-    end
-  end
-
-  ctx.part_folds[part_id] = new_folds
   ctx.global_folds = new_global
   output_window.set_folds(new_global)
 end

tests/unit/renderer_buffer_spec.lua

@@ -126,3 +126,67 @@ describe('renderer.buffer extmarks', function()
     assert_called_before(call_order, 'clear_extmarks', 'set_lines')
   end)
 end)
+
+describe('update_part_folds', function()
+  local set_folds_stub
+
+  before_each(function()
+    ctx:reset()
+    set_folds_stub = stub(output_window, 'set_folds')
+    ctx.global_folds = {}
+    ctx.part_folds = {}
+  end)
+
+  after_each(function()
+    set_folds_stub:revert()
+    ctx:reset()
+  end)
+
+  it('computes absolute fold ranges for a single part', function()
+    ctx.formatted_parts['part_a'] = {
+      lines = { 'title', '', 'content', 'more' },
+      fold_ranges = { { from = 1, to = 4 } },
+    }
+    ctx.render_state:set_part({ id = 'part_a', messageID = 'msg_1', type = 'text' }, 10, 14)
+
+    buffer.update_part_folds('part_a')
+
+    assert.stub(set_folds_stub).was_called_with({
+      { from = 10, to = 13 },
+    })
+  end)
+
+  it('skips set_folds when fold ranges have not changed', function()
+    ctx.formatted_parts['part_a'] = {
+      lines = { 'title', '', 'content', 'more' },
+      fold_ranges = { { from = 1, to = 4 } },
+    }
+    ctx.render_state:set_part({ id = 'part_a', messageID = 'msg_1', type = 'text' }, 10, 14)
+
+    buffer.update_part_folds('part_a')
+    set_folds_stub:clear()
+
+    buffer.update_part_folds('part_a')
+
+    assert.stub(set_folds_stub).was_not_called()
+  end)
+
+  it('merges existing folds from other parts', function()
+    ctx.part_folds['part_b'] = { { from = 5, to = 8 } }
+    ctx.global_folds = { { from = 5, to = 8 } }
+
+    ctx.formatted_parts['part_a'] = {
+      lines = { 'title', '', 'content', 'more' },
+      fold_ranges = { { from = 1, to = 4 } },
+    }
+    ctx.render_state:set_part({ id = 'part_a', messageID = 'msg_1', type = 'text' }, 10, 14)
+
+    buffer.update_part_folds('part_a')
+
+    assert.stub(set_folds_stub).was_called()
+    assert.stub(set_folds_stub).was_called_with({
+      { from = 5, to = 8 },
+      { from = 10, to = 13 },
+    })
+  end)
+end)