How to not have undo's across all Open Excels

AFAIK the undo is application wide and not specific to a workbook of worksheet. You might like to visit Feedback and make a suggestion to Microsoft about this issue. Following Microsoft link to guidelines to provide feedback.

https://support.microsoft.com/en-us/office/how-do-i-give-feedback-on-microsoft-office-2b102d44-b43f-4dd2-9ff4-23cf144cfb11

unless you are using another instance of excel on the workbook you're working with the undo only applies to that instance, for example you open five workbooks for reference purposes and then you open another instance of excel for the workbook that you're actually doing some changes or updates the undo only applies to that workbook.

unless you are using another instance of excel on the workbook you're working with the undo only applies to that instance, for example you open five workbooks for reference purposes and then you open another instance of excel for the workbook that you're actually doing some changes or updates the undo only applies to that workbook.

1. I open multiple workbooks (Let's call them Book1, Book2, Book3)

2. Activate Book2 and make a change.

3. Activate Book1 and make a change.

4. Activate Book3 and make a change.

5. Activate Book2 again and click Undo. Book3 is automatically activated and the previous change in Book3 is undone because that was the last change made.

6. Activate Book2 again and click Undo. Book1 is automatically activated and the previous change in Book1 is Undone

7. While Book1 is still activated, click Undo and Book2 is automatically activated and the change in Book2 is Undone.

I believe the above is what the OP is referring to but would prefer the Undo to only apply to the currently Active Workbook/Worksheet and ignore changes made in other Workbooks/Worksheets.

The following Microsoft Link describes how to edit the Registry so you can open Multiple instances of Excel so that the Undo is applied to each instance. However, I advise reading all of the article before attempting to implement because there is the possibility of having other undesired effects.

https://docs.microsoft.com/en-us/office/troubleshoot/excel/force-excel-to-open-new-instance

when I open a separate instance of excel usually the one I'll be working on after I open the first instance of excel, I press alt keep holding it, click start > click excel still holding alt until excel asks if I want to run a new (separate instance). I tested doing some actions then do the undo.

As a matter of fact where I worked before, the one instance of excel is running a slow **** macro so we could not do anything in that instance of excel so we had to run another (separate instance) so we won't have to deal with the slug instance running the macro.

You can just imagine one instance is running a macro and in the other instance (a separate thread) you're doing lots of undo action what do you think would happen to the instance running the macro?

when I open a separate instance of excel usually the one I'll be working on after I open the first instance of excel, I press alt keep holding it, click start > click excel still holding alt until excel asks if I want to run a new (separate instance). I tested doing some actions then do the undo.

As a matter of fact where I worked before, the one instance of excel is running a slow **** macro so we could not do anything in that instance of excel so we had to run another (separate instance) so we won't have to deal with the slug instance running the macro.

You can just imagine one instance is running a macro and in the other instance (a separate thread) you're doing lots of undo action what do you think would happen to the instance running the macro?

OK. I have learnt something new myself. Perhaps this will resolve the OP's problem but needs to be aware of the additional memory usage with separate instances of the application.

well you'll have to post that memory issue in a different community if it concerns you to run another instance so you can do some work while your other instance is running a macro, maybe uncle MS will listen.