diff --git a/.gitignore b/.gitignore
index 0f4ebc230..7a1916336 100644
--- a/.gitignore
+++ b/.gitignore
@@ -44,3 +44,6 @@ docs/_build
 
 # pycharm metadata
 .idea
+
+# VS Code
+.vscode/
diff --git a/examples/prompts/history/bug_thread_history.py b/examples/prompts/history/bug_thread_history.py
new file mode 100755
index 000000000..70685e4b2
--- /dev/null
+++ b/examples/prompts/history/bug_thread_history.py
@@ -0,0 +1,96 @@
+#!/usr/bin/env python
+"""
+Demonstrate bug in threaded history, where asynchronous loading can corrupt Buffer context.
+
+Seems to happen with very large history being loaded and causing slowdowns.
+
+"""
+import re
+import time
+
+from prompt_toolkit import PromptSession
+from prompt_toolkit.history import History, ThreadedHistory
+
+
+class MegaHistory(History):
+    """
+    Example class that loads lots of history
+
+    Sample designed to exercise existing multitasking hazards, don't add any more.
+    """
+
+    def __init__(self, init_request: int = 1000, *args, **kwargs):
+        super(MegaHistory, self).__init__(*args, **kwargs)
+        self.requested_count = 0  # only modified by main (requesting) thread
+        self.requested_commands = 0  # only modified by main (requesting) thread
+        self.actual_count = 0  # only modified by background thread
+
+    def load_history_strings(self):
+        while True:
+            while self.requested_count <= self.actual_count:
+                time.sleep(0.001)  # don't busy loop
+
+            print(
+                f"... starting to load {self.requested_count - self.actual_count:15,d} more items."
+            )
+            while self.requested_count > self.actual_count:
+                yield f"History item {self.actual_count:15,d}, command number {self.requested_commands}"
+                self.actual_count += 1
+            print("...done.")
+
+    def store_string(self, string):
+        pass  # Don't store strings.
+
+    # called by main thread, watch out for multitasking hazards.
+    def add_request(self, requested: int = 0):
+        self.requested_count += requested
+        self.requested_commands += 1
+
+    def show_request(self):
+        print(
+            f"Have loaded {self.actual_count:15,d} of {self.requested_count:15,d} in {self.requested_commands} commands."
+        )
+
+
+HIST_CMD = re.compile(r"^hist (load (\d+)|show)$", re.IGNORECASE)
+
+
+def main():
+    print(
+        "Asynchronous loading of history. Designed to exercise multitasking hazard in Buffer.\n"
+        "When started, tries to load 100,000 lines into history with no delay.\n"
+        "Expect to trigger assertion in document.py line 98, though others may fire.\n"
+        "\n"
+        "Can request more lines by `hist load nnnnn`, and can see progress by `hist show`.\n"
+        "\n"
+    )
+    mh = MegaHistory()
+    our_history = ThreadedHistory(mh)
+
+    # The history needs to be passed to the `PromptSession`. It can't be passed
+    # to the `prompt` call because only one history can be used during a
+    # session.
+    session = PromptSession(history=our_history)
+
+    mh.add_request(99999)
+
+    while True:
+        text = session.prompt("Say something: ")
+        if text.startswith("hist"):
+            m = HIST_CMD.match(text)
+            if not m:
+                print("eh?")
+            else:
+                if m[1] == "show":
+                    mh.show_request()
+                elif m[1].startswith("load"):
+                    mh.add_request(int(m[2]))
+                else:
+                    print("eh? hist load nnnnnn\nor hist show")
+            pass
+        else:
+            print("You said: %s" % text)
+
+
+if __name__ == "__main__":
+    main()
diff --git a/prompt_toolkit/buffer.py b/prompt_toolkit/buffer.py
index 6fea94014..49b7b152d 100644
--- a/prompt_toolkit/buffer.py
+++ b/prompt_toolkit/buffer.py
@@ -9,6 +9,7 @@
 import shutil
 import subprocess
 import tempfile
+import threading
 from enum import Enum
 from functools import wraps
 from typing import (
@@ -305,13 +306,13 @@ def __init__(
 
         # Load the history.
         def new_history_item(item: str) -> None:
-            # XXX: Keep in mind that this function can be called in a different
-            #      thread!
             # Insert the new string into `_working_lines`.
+            # XXX: This function contains a critical section, may only
+            #      be invoked on the event loop thread if history is
+            #      loading on another thread.
+
             self._working_lines.insert(0, item)
-            self.__working_index += (
-                1  # Not entirely threadsafe, but probably good enough.
-            )
+            self.__working_index += 1
 
         self.history.load(new_history_item)
 
@@ -413,6 +414,10 @@ def _set_cursor_position(self, value: int) -> bool:
 
     @property
     def text(self) -> str:
+        if self.working_index >= len(self._working_lines):
+            print(
+                f"Buffer: working_index {self.working_index} out of sync with working_lines {len(self._working_lines)}"
+            )
         return self._working_lines[self.working_index]
 
     @text.setter
diff --git a/prompt_toolkit/history.py b/prompt_toolkit/history.py
index 72acec9dc..cf5abd6a1 100644
--- a/prompt_toolkit/history.py
+++ b/prompt_toolkit/history.py
@@ -6,8 +6,10 @@
       when a history entry is loaded. This loading can be done asynchronously
       and making the history swappable would probably break this.
 """
+import asyncio
 import datetime
 import os
+import time
 from abc import ABCMeta, abstractmethod
 from threading import Thread
 from typing import Callable, Iterable, List, Optional
@@ -37,29 +39,12 @@ def __init__(self) -> None:
     # Methods expected by `Buffer`.
     #
 
-    def load(self, item_loaded_callback: Callable[[str], None]) -> None:
+    def load(self, item_loaded_callback: Callable[[str], None],) -> None:
         """
         Load the history and call the callback for every entry in the history.
+        This one assumes the callback is only called from same thread as `Buffer` is using.
 
-        XXX: The callback can be called from another thread, which happens in
-             case of `ThreadedHistory`.
-
-             We can't assume that an asyncio event loop is running, and
-             schedule the insertion into the `Buffer` using the event loop.
-
-             The reason is that the creation of the :class:`.History` object as
-             well as the start of the loading happens *before*
-             `Application.run()` is called, and it can continue even after
-             `Application.run()` terminates. (Which is useful to have a
-             complete history during the next prompt.)
-
-             Calling `get_event_loop()` right here is also not guaranteed to
-             return the same event loop which is used in `Application.run`,
-             because a new event loop can be created during the `run`. This is
-             useful in Python REPLs, where we want to use one event loop for
-             the prompt, and have another one active during the `eval` of the
-             commands. (Otherwise, the user can schedule a while/true loop and
-             freeze the UI.)
+        See `ThreadedHistory` for another way.
         """
         if self._loaded:
             for item in self._loaded_strings[::-1]:
@@ -123,26 +108,59 @@ def __init__(self, history: History) -> None:
         super().__init__()
 
     def load(self, item_loaded_callback: Callable[[str], None]) -> None:
+
+        """Collect the history strings on a background thread,
+        but run the callback in the event loop.
+
+        Caller of ThreadedHistory must ensure that the Application ends up running on the same
+        event loop as we (probably) create here.
+        """
+
         self._item_loaded_callbacks.append(item_loaded_callback)
 
+        def call_all_callbacks(item: str) -> None:
+            for cb in self._item_loaded_callbacks:
+                cb(item)
+
+        if self._loaded:  # ugly reference to base class internal...
+            for item in self._loaded_strings[::-1]:
+                call_all_callbacks(item)
+            return
+
         # Start the load thread, if we don't have a thread yet.
         if not self._load_thread:
 
-            def call_all_callbacks(item: str) -> None:
-                for cb in self._item_loaded_callbacks:
-                    cb(item)
+            event_loop = asyncio.get_event_loop()
 
             self._load_thread = Thread(
-                target=self.history.load, args=(call_all_callbacks,)
+                target=self.bg_loader, args=(call_all_callbacks, event_loop)
             )
             self._load_thread.daemon = True
             self._load_thread.start()
 
-    def get_strings(self) -> List[str]:
-        return self.history.get_strings()
+    def bg_loader(
+        self,
+        item_loaded_callback: Callable[[str], None],
+        event_loop: asyncio.BaseEventLoop,
+    ) -> None:
+        """
+        Load the history and schedule the callback for every entry in the history.
+        TODO: extend the callback so it can take a batch of lines in one event_loop dispatch.
+        """
 
-    def append_string(self, string: str) -> None:
-        self.history.append_string(string)
+        try:
+            for item in self.load_history_strings():
+                self._loaded_strings.insert(
+                    0, item
+                )  # slowest way to add an element to a list known to man.
+                event_loop.call_soon_threadsafe(
+                    item_loaded_callback, item
+                )  # expensive way to dispatch single line.
+        finally:
+            self._loaded = True
+
+    def __repr__(self) -> str:
+        return "ThreadedHistory(%r)" % (self.history,)
 
     # All of the following are proxied to `self.history`.
 
@@ -152,9 +170,6 @@ def load_history_strings(self) -> Iterable[str]:
     def store_string(self, string: str) -> None:
         self.history.store_string(string)
 
-    def __repr__(self) -> str:
-        return "ThreadedHistory(%r)" % (self.history,)
-
 
 class InMemoryHistory(History):
     """