event: No longer process K_EVENT automatically

Two new functions, `event_enable_deferred()`/`event_disable_deferred()` have to
be called by code that is capable of handling asynchronicity. User-dialog states
like "press ENTER to continue" or the swap file confirmation no longer will
generate K_EVENT.

src/nvim/edit.c

@@ -593,9 +593,17 @@ edit (
      * Get a character for Insert mode.  Ignore K_IGNORE.
      */
     lastc = c;                          /* remember previous char for CTRL-D */
+    event_enable_deferred();
     do {
       c = safe_vgetc();
     } while (c == K_IGNORE);
+    event_disable_deferred();
+
+    if (c == K_EVENT) {
+      c = lastc;
+      event_process();
+      continue;
+    }
 
     /* Don't want K_CURSORHOLD for the second key, e.g., after CTRL-V. */
     did_cursorhold = TRUE;
@@ -943,10 +951,6 @@ doESCkey:
       did_cursorhold = TRUE;
       break;
 
-    case K_EVENT:
-      event_process();
-      break;
-
     case K_HOME:        /* <Home> */
     case K_KHOME:
     case K_S_HOME:

src/nvim/ex_getln.c

@@ -311,9 +311,16 @@ getcmdline (
 
     /* Get a character.  Ignore K_IGNORE, it should not do anything, such
      * as stop completion. */
+    event_enable_deferred();
     do {
       c = safe_vgetc();
     } while (c == K_IGNORE);
+    event_disable_deferred();
+
+    if (c == K_EVENT) {
+      event_process();
+      continue;
+    }
 
     if (KeyTyped) {
       some_key_typed = TRUE;
@@ -769,11 +776,6 @@ getcmdline (
      * Big switch for a typed command line character.
      */
     switch (c) {
-    case K_EVENT:
-      event_process();
-      // Force a redraw even though the command line didn't change
-      shell_resized();
-      goto cmdline_not_changed;
     case K_BS:
     case Ctrl_H:
     case K_DEL:
@@ -1885,8 +1887,6 @@ redraw:
       }
 
       if (IS_SPECIAL(c1)) {
-        // Process deferred events
-        event_process();
         // Ignore other special key codes
         continue;
       }

src/nvim/getchar.c

@@ -2481,7 +2481,6 @@ inchar (
       char_u dum[DUM_LEN + 1];
 
       for (;; ) {
-        event_process();
         len = ui_inchar(dum, DUM_LEN, 0L, 0);
         if (len == 0 || (len == 1 && dum[0] == 3))
           break;

src/nvim/message.c

@@ -44,7 +44,6 @@
 #include "nvim/term.h"
 #include "nvim/ui.h"
 #include "nvim/os/os.h"
-#include "nvim/os/event.h"
 
 /*
  * To be able to scroll back at the "more" and "hit-enter" prompts we need to
@@ -2076,9 +2075,6 @@ static int do_more_prompt(int typed_char)
 
     toscroll = 0;
     switch (c) {
-    case K_EVENT:
-      event_process();
-      break;
     case BS:                    /* scroll one line back */
     case K_BS:
     case 'k':
@@ -2738,8 +2734,6 @@ do_dialog (
       break;
     default:                  /* Could be a hotkey? */
       if (c < 0) {            /* special keys are ignored here */
-        // drain event queue to prevent infinite loop
-        event_process();
         continue;
       }
       if (c == ':' && ex_cmd) {

src/nvim/normal.c

@@ -312,7 +312,6 @@ static const struct nv_cmd {
   {K_F8,      farsi_fkey,     0,                      0},
   {K_F9,      farsi_fkey,     0,                      0},
   {K_CURSORHOLD, nv_cursorhold, NV_KEEPREG,           0},
-  {K_EVENT,   nv_event,       NV_KEEPREG,             0},
 };
 
 /* Number of commands in nv_cmds[]. */
@@ -483,7 +482,15 @@ normal_cmd (
   /*
    * Get the command character from the user.
    */
+  event_enable_deferred();
   c = safe_vgetc();
+  event_disable_deferred();
+
+  if (c == K_EVENT) {
+    event_process();
+    return;
+  }
+
   LANGMAP_ADJUST(c, true);
 
   /*
@@ -7373,8 +7380,3 @@ static void nv_cursorhold(cmdarg_T *cap)
   did_cursorhold = true;
   cap->retval |= CA_COMMAND_BUSY;       /* don't call edit() now */
 }
-
-static void nv_event(cmdarg_T *cap)
-{
-  event_process();
-}

src/nvim/os/event.c

@@ -18,6 +18,8 @@
 #include "nvim/vim.h"
 #include "nvim/memory.h"
 #include "nvim/misc2.h"
+#include "nvim/term.h"
+#include "nvim/screen.h"
 
 #include "nvim/lib/klist.h"
 
@@ -39,6 +41,7 @@ typedef struct {
 //                   loop(to avoid recursion), but before returning from
 //                   `event_poll`
 static klist_t(Event) *deferred_events = NULL, *immediate_events = NULL;
+static int deferred_events_allowed = 0;
 
 void event_init(void)
 {
@@ -134,7 +137,17 @@ void event_poll(int ms)
 
 bool event_has_deferred(void)
 {
-  return !kl_empty(deferred_events);
+  return deferred_events_allowed && !kl_empty(deferred_events);
+}
+
+void event_enable_deferred(void)
+{
+  ++deferred_events_allowed;
+}
+
+void event_disable_deferred(void)
+{
+  --deferred_events_allowed;
 }
 
 // Queue an event
@@ -146,6 +159,11 @@ void event_push(Event event, bool deferred)
 void event_process(void)
 {
   process_events_from(deferred_events);
+
+  if (must_redraw) {
+    update_screen(0);
+    out_flush();
+  }
 }
 
 static void process_events_from(klist_t(Event) *queue)