[bug] Different bundle hash in clean project and subsequent builds (EventSystem ExecutionOrder bug)

I've reported it as Editor's bug (1220916), but it affects bundles building with included EventSystem, so repeating it here.

Spoiler: 1220916 description

EventSystem's ExecutionOrder is wrong (0 instead of -1000) when opening project without Library/
*It also affects to results of building the player and bundles (bundle's hash depends on ExecutionOrder written).*

== Steps to reproduce
1. Open clean project Bug-EventSystem-ExecutionOrder-2018 (without Library/ in its folder) in Unity 2018.4.*
2. Error message in console: "!!! ERROR: EventSystem (MonoScript) execution order: 0 (current) != -1000 (required) !!!"
3. Close Unity and open project again.
5. Message: "EventSystem (MonoScript) execution order: -1000 - OK"
--
*. You can reproduce by opening project after removing Library/ folder again.
**. You can repeate checking in editor via menu command: [TEST]->Check EventSystem Exection Order

Reproduced in 2018.4.16f1 and .2f1 (mac/linux/windows) and bug seems to be old (https://forum.unity.com/threads/eventsystem-script-execution-order-changed-in-2018-3-0b11.588364/)
It isn't reproduced in 2019.3.0f6 (with asset pipeline v2).

Workaround:
Preliminary Unity opening (e.g. with -quit) when building bundles in clean project.

 

Unfortunately yes - it's the engine bug.
I just want to warn people until it's not fixed:
Don't build bundles immediately after clean checkout! (e.g. on build-agents)
First - open project and close it and only after that - execute the build process.

 

Workaround without preliminary project opening - fix EventSystem's execution order via InitializeOnLoad:

Code (CSharp):

1. #if !UNITY_2019_3_OR_NEWER //already fixed
2. using System.Linq;
3. using UnityEditor;
4. using UnityEngine;
5.  
6. /// <summary>
7. /// Workaround for incorrect EventSystem execution order opening project and building player/bundles without Library in gui/batchmode (1220916)
8. ///     https://forum.unity.com/threads/bug-different-bundle-hash-in-clean-project-and-subsequent-builds-eventsystem-executionorder-bug.830892/
9. /// </summary>
10. [InitializeOnLoad]
11. public static class FixEventSystemExecutionOrder
12. {
13.     static FixEventSystemExecutionOrder() => EnsureMonoScriptOrder("EventSystem", -1000);
14.  
15.     private static void EnsureMonoScriptOrder(string scriptName = null, int requiredOrder = 0)
16.     {
17.         var scripts = MonoImporter.GetAllRuntimeMonoScripts().Where(x => x.name == scriptName);
18.         foreach (var script in scripts) {
19.             var order = MonoImporter.GetExecutionOrder(script);
20.             if (order == requiredOrder) continue;
21.             Debug.LogWarning($"Fixing {script.name} [{script.GetInstanceID()}] execution order [{order} (current) != {requiredOrder} (required)]");
22.             MonoImporter.SetExecutionOrder(script, requiredOrder);
23.         }
24.     }
25. }
26. #endif
27.