eventloop: K_EVENT should not finish operator

normal_finish_command() and normal_prepare() assume that any pending
operator needs to be finished after any subsequent key.

Set `finish_op = false` in nv_event() to indicate that the pending
operator shouldn't be finished in normal_execute().

This is how nv_visual() indicates that 'v' or 'V' in operator-pending
mode should not finish the current pending operator.

fixes #5398
fixes #6166 (partially; mappings are still interrupted)

src/nvim/normal.c

@@ -7958,6 +7958,7 @@ static void nv_event(cmdarg_T *cap)
   may_garbage_collect = false;
   multiqueue_process_events(main_loop.events);
   cap->retval |= CA_COMMAND_BUSY;       // don't call edit() now
+  finish_op = false;
 }
 
 /// Trigger FocusGained event.

test/functional/api/vim_spec.lua

@@ -329,24 +329,80 @@ describe('api', function()
       }
       eq({ { {mode='n', blocking=false},
              13,
-             {mode='n', blocking=false},  -- TODO: should be blocked=true
+             {mode='n', blocking=false},  -- TODO: should be blocked=true ?
              1 },
            NIL}, meths.call_atomic(req))
       eq({mode='r', blocking=true}, nvim("get_mode"))
     end)
-    -- TODO: bug #6166
     it("during insert-mode map-pending, returns blocking=true #6166", function()
       command("inoremap xx foo")
       nvim("input", "ix")
       eq({mode='i', blocking=true}, nvim("get_mode"))
     end)
-    -- TODO: bug #6166
     it("during normal-mode gU, returns blocking=false #6166", function()
       nvim("input", "gu")
       eq({mode='no', blocking=false}, nvim("get_mode"))
     end)
   end)
 
+  describe('RPC (K_EVENT) #6166', function()
+    it('does not complete/interrupt normal-mode operator', function()
+      helpers.insert([[
+        FIRST LINE
+        SECOND LINE]])
+      nvim('input', 'gg')
+      nvim('input', 'gu')
+      -- Make any non-async RPC request.
+      nvim('get_current_buf')
+      -- Buffer should not change.
+      helpers.expect([[
+        FIRST LINE
+        SECOND LINE]])
+      -- Now send input to complete the operator.
+      nvim("input", "j")
+      helpers.expect([[
+      first line
+      second line]])
+    end)
+    -- TODO: bug #6166
+    pending('does not complete/interrupt normal-mode mapping', function()
+      command("nnoremap dd :let g:foo='it worked...'<CR>")
+      helpers.insert([[
+        FIRST LINE
+        SECOND LINE]])
+      nvim('input', 'gg')
+      nvim('input', 'd')
+      helpers.expect([[
+        FIRST LINE
+        SECOND LINE]])
+      -- Make any non-async RPC request. (expect() does RPC, but be explicit)
+      nvim('get_current_buf')
+      -- Send input to complete the mapping.
+      nvim('input', 'd')
+      helpers.expect([[
+        FIRST LINE
+        SECOND LINE]])
+      eq('it worked...', eval('g:foo'))
+    end)
+    it('does not complete/interrupt insert-mode mapping', function()
+      command("inoremap xx foo")
+      helpers.insert([[
+        FIRST LINE
+        SECOND LINE]])
+      nvim('input', 'ix')
+      helpers.expect([[
+        FIRST LINE
+        SECOND LINxE]])
+      -- Make any non-async RPC request. (expect() does RPC, but be explicit)
+      nvim('get_current_buf')
+      -- Send input to complete the mapping.
+      nvim('input', 'x')
+      helpers.expect([[
+        FIRST LINE
+        SECOND LINxxE]])  -- TODO: should be "SECOND LINfooE" #6166
+    end)
+  end)
+
   describe('nvim_replace_termcodes', function()
     it('escapes K_SPECIAL as K_SPECIAL KS_SPECIAL KE_FILLER', function()
       eq('\128\254X', helpers.nvim('replace_termcodes', '\128', true, true, true))