fix(event-loop): process input before events (#27358)

Problem:
When nvim_input is followed immediately by non-fast events on RPC, both
events and input are available after the polling done by the os_inchar()
in state_enter(), but state_enter() then chooses to process events even
if input is available, which is inconsistent with state_handle_k_event()
that stops processing events once input is available.

Solution:
Also check for available input after the os_inchar() in state_enter().

2 files changed, 9 insertions, 1 deletions

diff --git a/src/nvim/state.c b/src/nvim/state.c
index 527acee306..0df060ecf4 100644
--- a/src/nvim/state.c
+++ b/src/nvim/state.c
@@ -79,7 +79,7 @@ getkey:
       // mapping engine.
       os_inchar(NULL, 0, -1, typebuf.tb_change_cnt, main_loop.events);
       // If an event was put into the queue, we send K_EVENT directly.
-      if (!multiqueue_empty(main_loop.events)) {
+      if (!input_available() && !multiqueue_empty(main_loop.events)) {
         key = K_EVENT;
       } else {
         goto getkey;
diff --git a/test/functional/api/vim_spec.lua b/test/functional/api/vim_spec.lua
index 5cf48412a8..4cd46f5e91 100644
--- a/test/functional/api/vim_spec.lua
+++ b/test/functional/api/vim_spec.lua
@@ -95,6 +95,14 @@ describe('API', function()
     assert_alive()
   end)
 
+  it('input is processed first when followed immediately by non-fast events', function()
+    api.nvim_set_current_line('ab')
+    async_meths.nvim_input('x')
+    async_meths.nvim_exec_lua('_G.res1 = vim.api.nvim_get_current_line()', {})
+    async_meths.nvim_exec_lua('_G.res2 = vim.api.nvim_get_current_line()', {})
+    eq({ 'b', 'b' }, exec_lua('return { _G.res1, _G.res2 }'))
+  end)
+
   it('does not set CA_COMMAND_BUSY #7254', function()
     command('split')
     command('autocmd WinEnter * startinsert')