main: Refactor normal_enter to call os_inchar directly

This makes it impossible for K_EVENT to interfere with mappings, but it also
disables processing of events while in the middle of a mapping (Though this will
be fixed later as this refactoring progresses).

`may_sync_undo` is now called when K_EVENT is received. This is necessary to
correctly update undo entry lists before executing some action.

src/nvim/getchar.c

@@ -1131,7 +1131,7 @@ static void gotchars(char_u *chars, int len)
  * - While reading a script file.
  * - When no_u_sync is non-zero.
  */
-static void may_sync_undo(void)
+void may_sync_undo(void)
 {
   if ((!(State & (INSERT + CMDLINE)) || arrow_used)
       && scriptin[curscript] == NULL)

src/nvim/normal.c

@@ -464,10 +464,34 @@ void normal_enter(bool cmdwin, bool noexmode)
       continue;
     }
 
-    input_enable_events();
-    int c = safe_vgetc();
-    input_disable_events();
-    if (!normal_execute(&state, c)) {
+    int key;
+
+    if (char_avail() || using_script() || input_available()) {
+      // Don't block for events if there's a character already available for
+      // processing. Characters can come from mappings, scripts and other
+      // sources, so this scenario is very common.
+      key = safe_vgetc();
+    } else if (!queue_empty(loop.events)) {
+      // Event was made available after the last queue_process_events call
+      key = K_EVENT;
+    } else {
+      input_enable_events();
+      // Flush screen updates before blocking
+      ui_flush();
+      // Call `os_inchar` directly to block for events or user input without
+      // consuming anything from `input_buffer`(os/input.c) or calling the
+      // mapping engine. If an event was put into the queue, we send K_EVENT
+      // directly.
+      (void)os_inchar(NULL, 0, -1, 0);
+      input_disable_events();
+      key = !queue_empty(loop.events) ? K_EVENT : safe_vgetc();
+    }
+
+    if (key == K_EVENT) {
+      may_sync_undo();
+    }
+
+    if (!normal_execute(&state, key)) {
       break;
     }
   }

src/nvim/os/input.c

@@ -91,7 +91,7 @@ static void create_cursorhold_event(void)
 // Low level input function
 int os_inchar(uint8_t *buf, int maxlen, int ms, int tb_change_cnt)
 {
-  if (rbuffer_size(input_buffer)) {
+  if (maxlen && rbuffer_size(input_buffer)) {
     return (int)rbuffer_read(input_buffer, (char *)buf, (size_t)maxlen);
   }
 
@@ -116,14 +116,14 @@ int os_inchar(uint8_t *buf, int maxlen, int ms, int tb_change_cnt)
     return 0;
   }
 
-  if (rbuffer_size(input_buffer)) {
+  if (maxlen && rbuffer_size(input_buffer)) {
     // Safe to convert rbuffer_read to int, it will never overflow since we use
     // relatively small buffers.
     return (int)rbuffer_read(input_buffer, (char *)buf, (size_t)maxlen);
   }
 
   // If there are events, return the keys directly
-  if (pending_events()) {
+  if (maxlen && pending_events()) {
     return push_event_key(buf, maxlen);
   }
 
@@ -324,6 +324,11 @@ void input_done(void)
   input_eof = true;
 }
 
+bool input_available(void)
+{
+  return rbuffer_size(input_buffer) != 0;
+}
+
 // This is a replacement for the old `WaitForChar` function in os_unix.c
 static InbufPollResult inbuf_poll(int ms)
 {