Fix: Undo/redo cursor position fixed

[Alspb]

Description

Fixed cursor position after Undo/Redo.

Related Issues

Fixes #1289, #1005.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the package version in pubspec.yaml files.
• All existing and new tests are passing.
• I have run the commands in ./scripts/before_push.sh and it all passed successfully

Breaking Change

Does your PR require developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[ellet0]

Thank you for your contributions

Not related to the PR: I noticed the names we use for the variables are very short and can be unclear for developers who are new to the project. We might want to use longer and more descriptive names

For example,len might or might not indicate its length also, it's not clear length of what exactly, or something like v which is not clear to most developers

Thank you once again.

[Alspb]

I agree.
In this particular PR I tried to preserve the names but probably it's better to change them when working on a PR.