feat: trigger ModeChanged for terminal modes

src/nvim/terminal.c

@@ -412,6 +412,7 @@ void terminal_enter(void)
   curwin->w_redr_status = true;  // For mode() in statusline. #8323
   ui_busy_start();
   apply_autocmds(EVENT_TERMENTER, NULL, NULL, false, curbuf);
+  trigger_modechanged();
 
   s->state.execute = terminal_execute;
   s->state.check = terminal_check;

test/functional/autocmd/modechanged_spec.lua

@@ -0,0 +1,31 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear, eval, eq = helpers.clear, helpers.eval, helpers.eq
+local feed, command = helpers.feed, helpers.command
+
+describe('ModeChanged', function()
+  before_each(function()
+    clear()
+    command('let g:count = 0')
+    command('au ModeChanged * let g:event = copy(v:event)')
+    command('au ModeChanged * let g:count += 1')
+  end)
+
+  it('picks up terminal mode changes', function()
+    command("term")
+    feed('i')
+    eq({
+      old_mode = 'nt',
+      new_mode = 't'
+    }, eval('g:event'))
+    feed('<c-\\><c-n>')
+    eq({
+      old_mode = 't',
+      new_mode = 'nt'
+    }, eval('g:event'))
+    eq(3, eval('g:count'))
+    command("bd!")
+
+    -- v:event is cleared after the autocommand is done
+    eq({}, eval('v:event'))
+  end)
+end)