fix(jobs): do not block UI when jobwait() doesn't block (#31803)

(cherry picked from commit efe1732c6f)

src/nvim/eval/funcs.c

@@ -4562,8 +4562,6 @@ static void f_jobwait(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     return;
   }
 
-  ui_busy_start();
-  ui_flush();
   list_T *args = argvars[0].vval.v_list;
   Channel **jobs = xcalloc((size_t)tv_list_len(args), sizeof(*jobs));
   MultiQueue *waiting_jobs = multiqueue_new_parent(loop_on_put, &main_loop);
@@ -4600,6 +4598,13 @@ static void f_jobwait(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
     before = os_hrtime();
   }
 
+  // Only mark the UI as busy when jobwait() blocks
+  const bool busy = remaining != 0;
+  if (busy) {
+    ui_busy_start();
+    ui_flush();
+  }
+
   for (i = 0; i < tv_list_len(args); i++) {
     if (remaining == 0) {
       break;  // Timeout.
@@ -4641,7 +4646,9 @@ static void f_jobwait(typval_T *argvars, typval_T *rettv, EvalFuncData fptr)
 
   multiqueue_free(waiting_jobs);
   xfree(jobs);
-  ui_busy_stop();
+  if (busy) {
+    ui_busy_stop();
+  }
   tv_list_ref(rv);
   rettv->v_type = VAR_LIST;
   rettv->vval.v_list = rv;

test/functional/core/job_spec.lua

@@ -943,6 +943,40 @@ describe('jobs', function()
       feed('<CR>')
       fn.jobstop(api.nvim_get_var('id'))
     end)
+
+    it('does not set UI busy with zero timeout #31712', function()
+      local screen = Screen.new(50, 6)
+      screen:attach()
+      command([[let g:id = jobstart(['sleep', '0.3'])]])
+      local busy = 0
+      screen._handle_busy_start = (function(orig)
+        return function()
+          orig(screen)
+          busy = busy + 1
+        end
+      end)(screen._handle_busy_start)
+      source([[
+        func PrintAndPoll()
+          echon "aaa\nbbb"
+          call jobwait([g:id], 0)
+          echon "\nccc"
+        endfunc
+      ]])
+      feed_command('call PrintAndPoll()')
+      screen:expect {
+        grid = [[
+                                                          |
+        {3:                                                  }|
+        aaa                                               |
+        bbb                                               |
+        ccc                                               |
+        {6:Press ENTER or type command to continue}^           |
+      ]],
+      }
+      feed('<CR>')
+      fn.jobstop(api.nvim_get_var('id'))
+      eq(0, busy)
+    end)
   end)
 
   pending('exit event follows stdout, stderr', function()