fix(lsp): requery empty isIncomplete completion lists #40100

Problem:
an empty `{ isIncomplete = true, items = {} }` ends completion
instead of requerying.

Solution:
keep isIncomplete on empty lists and retrigger on keypress while incomplete.
Reset on <C-e> so it doesn't immediately re-query.

runtime/lua/vim/lsp/completion.lua

@@ -937,6 +937,9 @@ local function register_completedone(bufnr)
     local reason = api.nvim_get_vvar('event').reason ---@type string
     if reason == 'accept' then
       on_complete_done()
+    elseif reason == 'cancel' then
+      -- <C-e> dismissed the pum; stop re-querying an incomplete list.
+      Context:reset()
     end
   end)
 
@@ -1002,23 +1005,27 @@ local function trigger(bufnr, clients, ctx)
             client and client.name or 'UNKNOWN'
           )
         )
-      elseif not vim.isnil(result) and #(result.items or result) > 0 then
+      -- result is CompletionItem[] or CompletionList; result.items may be empty, and
+      -- an empty incomplete list means request again when needed.
+      elseif not vim.isnil(result) then
         Context.isIncomplete = Context.isIncomplete or result.isIncomplete
-        local encoding = client and client.offset_encoding or 'utf-16'
-        local client_matches, tmp_server_start_boundary
-        client_matches, tmp_server_start_boundary = M._convert_results(
-          line,
-          cursor_row - 1,
-          cursor_col,
-          client_id,
-          word_boundary,
-          nil,
-          result,
-          encoding
-        )
+        if #(result.items or result) > 0 then
+          local encoding = client and client.offset_encoding or 'utf-16'
+          local client_matches, tmp_server_start_boundary
+          client_matches, tmp_server_start_boundary = M._convert_results(
+            line,
+            cursor_row - 1,
+            cursor_col,
+            client_id,
+            word_boundary,
+            nil,
+            result,
+            encoding
+          )
 
-        server_start_boundary = tmp_server_start_boundary or server_start_boundary
-        vim.list_extend(matches, client_matches)
+          server_start_boundary = tmp_server_start_boundary or server_start_boundary
+          vim.list_extend(matches, client_matches)
+        end
       end
     end
 
@@ -1056,31 +1063,33 @@ end
 
 --- @param handle vim.lsp.completion.BufHandle
 local function on_insert_char_pre(handle)
-  if vim.fn.pumvisible() ~= 0 then
-    if Context.isIncomplete then
-      reset_timer()
+  if Context.isIncomplete then
+    reset_timer()
 
-      local debounce_ms = adaptive_debounce(Context.last_request_time, rtt_ms)
-      local ctx = { triggerKind = protocol.CompletionTriggerKind.TriggerForIncompleteCompletions }
-      if debounce_ms == 0 then
-        vim.schedule(function()
+    local debounce_ms = adaptive_debounce(Context.last_request_time, rtt_ms)
+    local ctx = { triggerKind = protocol.CompletionTriggerKind.TriggerForIncompleteCompletions }
+    if debounce_ms == 0 then
+      vim.schedule(function()
+        M.get({ ctx = ctx })
+      end)
+    else
+      completion_timer = new_timer()
+      completion_timer:start(
+        math.floor(debounce_ms),
+        0,
+        vim.schedule_wrap(function()
           M.get({ ctx = ctx })
         end)
-      else
-        completion_timer = new_timer()
-        completion_timer:start(
-          math.floor(debounce_ms),
-          0,
-          vim.schedule_wrap(function()
-            M.get({ ctx = ctx })
-          end)
-        )
-      end
+      )
     end
 
     return
   end
 
+  if vim.fn.pumvisible() ~= 0 then
+    return
+  end
+
   local char = vim.v.char
   local matched_clients = handle.triggers[char]
   -- Discard pending trigger char, complete the "latest" one.

test/functional/plugin/lsp/completion_spec.lua

@@ -1241,6 +1241,44 @@ describe('vim.lsp.completion: protocol', function()
     end)
     t.matches('items=null', err)
   end)
+
+  it('keeps requerying while the completion list is incomplete #40096', function()
+    exec_lua(function()
+      _G.contexts = {}
+      local server = _G._create_server({
+        capabilities = {
+          completionProvider = { triggerCharacters = { 'h' } },
+        },
+        handlers = {
+          ['textDocument/completion'] = function(_, params, callback)
+            _G.contexts[#_G.contexts + 1] = params.context
+            callback(nil, { isIncomplete = true, items = { { label = 'hello' } } })
+          end,
+        },
+      })
+      local bufnr = vim.api.nvim_get_current_buf()
+      vim.api.nvim_win_set_buf(0, bufnr)
+      vim.lsp.start({
+        name = 'dummy',
+        cmd = server.cmd,
+        on_attach = function(client, bufnr0)
+          vim.lsp.completion.enable(true, client.id, bufnr0, { autotrigger = true })
+        end,
+      })
+    end)
+    feed('ih')
+    assert_matches(function(matches)
+      eq('hello', matches[1].word)
+    end)
+    eq({ triggerKind = 2, triggerCharacter = 'h' }, exec_lua('return _G.contexts[1]'))
+
+    exec_lua('_G.capture = {}')
+    feed('e')
+    assert_matches(function(matches)
+      eq('hello', matches[1].word)
+    end)
+    eq({ triggerKind = 3 }, exec_lua('return _G.contexts[2]'))
+  end)
 end)
 
 describe('vim.lsp.completion: integration', function()