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/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;
     }
   }