feat(lsp): improve signature help display #35190

- Add delimiter between function signature and documentation, matching hover formatting
- Show title only if there are multiple clients or multiple signatures
- Avoid duplicating the title inside the window if it's already shown in the border

(cherry picked from commit d26db4bfbf)

runtime/lua/vim/lsp/buf.lua

@@ -386,17 +386,21 @@ function M.signature_help(config)
         return
       end
 
+      -- Show title only if there are multiple clients or multiple signatures.
+      if total > 1 then
         local sfx = total > 1
             and string.format(' (%d/%d)%s', idx, total, can_cycle and ' (<C-s> to cycle)' or '')
           or ''
         config.title = config.title or string.format('Signature Help: %s%s', client.name, sfx)
-      if not config.border then
+        -- If no border is set, render title inside the window.
+        if not (config.border or vim.o.winborder ~= '') then
           table.insert(lines, 1, '# ' .. config.title)
           if hl then
             hl[1] = hl[1] + 1
             hl[3] = hl[3] + 1
           end
         end
+      end
 
       config._update_win = update_win
 

runtime/lua/vim/lsp/util.lua

@@ -745,6 +745,10 @@ function M.convert_signature_help_to_markdown_lines(signature_help, ft, triggers
     if type(doc) == 'string' then
       signature.documentation = { kind = 'plaintext', value = doc }
     end
+    -- Add delimiter if there is documentation to display
+    if signature.documentation.value ~= '' then
+      contents[#contents + 1] = '---'
+    end
     M.convert_input_to_markdown_lines(signature.documentation, contents)
   end
   if signature.parameters and #signature.parameters > 0 then

test/functional/plugin/lsp_spec.lua

@@ -3657,7 +3657,7 @@ describe('LSP', function()
         }
         return vim.lsp.util.convert_signature_help_to_markdown_lines(signature_help, 'cs', { ',' })
       end)
-      local expected = { '```cs', 'TestEntity.TestEntity()', '```', 'some doc' }
+      local expected = { '```cs', 'TestEntity.TestEntity()', '```', '---', 'some doc' }
       eq(expected, result)
     end)