backport release-23.05: helix: fix UB in diff gutter

[atalii]

Description of changes

Applies the fix from #249163 to the release branch. This had to be done manually due to some merge conflicts.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.11 Release Notes (or backporting 23.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

---

Could someone confirm that this definitely does alleviate the bug when cherry-picked to this branch?

[atalii]

Sorry for the ping, but anyone okay to review or merge? It's been roughly a week; to my understanding this issue still affects stable? @danth @yusdacra @zowoq

[danbergelt]

Hi! Following up on this, would love to see this behavior patched on stable :)