fix: prevent K_EVENT from stopping Select mode CTRL-O #15688

When using Goneovim, Select mode `CTRL-O` returns back to Select mode
immediately (even with `--clean`). Neovim TUI (with some plugins) also randomly
returns to Select mode even if no keys are pressed when using `CTRL-O` in Select
mode.

test/functional/visual/ctrl_o_spec.lua

@@ -0,0 +1,23 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local eq = helpers.eq
+local eval = helpers.eval
+local expect = helpers.expect
+local feed = helpers.feed
+local meths = helpers.meths
+
+describe('select-mode Ctrl-O', function()
+  before_each(clear)
+
+  it("doesn't cancel Ctrl-O mode when processing event", function()
+    feed('iHello World<esc>gh<c-o>')
+    eq({mode='vs', blocking=false}, meths.get_mode()) -- fast event
+    eq(2, eval('1+1'))  -- causes K_EVENT key
+    eq({mode='vs', blocking=false}, meths.get_mode()) -- still in ctrl-o mode
+    feed('^')
+    eq({mode='s', blocking=false}, meths.get_mode()) -- left ctrl-o mode
+    feed('h')
+    eq({mode='i', blocking=false}, meths.get_mode()) -- entered insert mode
+    expect('h') -- selection is the whole line and is replaced
+  end)
+end)