Use fixed positioning for ha-form-multi_select

[karwosts]

Proposed change

Attempt to fix ha-form-multi_select used in a dialog. Apparently it's been not behaving well since it was updated from MD2 to MD3.

Changing the positioning to fixed seems to behave more like as would be expected. I cannot imagine every possible usecase of this form element, so I can't test that this works correctly in every case.

Unsure if using fixed positioning could have any drawbacks. Unsure if I should try to limit the positioning change to some subset of instances (in dialogs only?). It looks fine with this change in time-condition which is the other usecase I know of.

Current (using github integration options flow as example):

With fix:

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes Homekit dialog is cut-off #23583
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io