eventloop: FocusGained: schedule event instead of pseudokey

closes #4840
closes #6164

src/nvim/aucmd.c

@@ -0,0 +1,48 @@
+// This is an open source non-commercial project. Dear PVS-Studio, please check
+// it. PVS-Studio Static Code Analyzer for C, C++ and C#: http://www.viva64.com
+
+#include "nvim/os/os.h"
+#include "nvim/fileio.h"
+#include "nvim/vim.h"
+#include "nvim/main.h"
+#include "nvim/screen.h"
+#include "nvim/ui.h"
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "aucmd.c.generated.h"
+#endif
+
+static void focusgained_event(void **argv)
+{
+  bool *gained = argv[0];
+  do_autocmd_focusgained(*gained);
+  xfree(gained);
+}
+void aucmd_schedule_focusgained(bool gained)
+{
+  bool *gainedp = xmalloc(sizeof(*gainedp));
+  *gainedp = gained;
+  loop_schedule(&main_loop, event_create(focusgained_event, 1, gainedp));
+}
+
+static void do_autocmd_focusgained(bool gained)
+  FUNC_ATTR_NONNULL_ALL
+{
+  static bool recursive = false;
+
+  if (recursive) {
+    return;  // disallow recursion
+  }
+  recursive = true;
+  bool has_any = has_event(EVENT_FOCUSGAINED) || has_event(EVENT_FOCUSLOST);
+  bool did_any = apply_autocmds((gained ? EVENT_FOCUSGAINED : EVENT_FOCUSLOST),
+                                NULL, NULL, false, curbuf);
+  if (has_any && !did_any) {
+    // HACK: Reschedule, hoping that the next event-loop tick will pick this up
+    // during a "regular" state (as opposed to a weird implicit state, e.g.
+    // early_init()..win_alloc_first() which disables autocommands).
+    aucmd_schedule_focusgained(gained);
+  }
+  recursive = false;
+}
+