[Grid Layout / Flyout System] When sidebar is visible, child flyouts have wrong positioning

[tsullivan]

Kibana version:
This bug transpires using an unreleased version of the EUI Flyout System. Feature branch PR.

Elasticsearch version:

Server OS version:

Browser version:
Chrome 142.0.7444.60 (Official Build) (arm64)

Browser OS version:
macOS Tahoe 26.1

Original install method (e.g. download page, yum, from source, etc.):
From source

Describe the bug:
The Flyout System positions "child" flyouts at an offset which is equal to the width of its parent. This offset calculation is wrong in the Grid Layout when testing with core.chrome.layoutDebug: true

Steps to reproduce:

1. Flyout System Example in: Example of Flyout System in Kibana Example #233806
2. Set in config/kibana.dev.yml:

    feature_flags.overrides:
      core.chrome.layoutType: 'grid'
      core.chrome.layoutDebug: true
   

3. Note how child flyouts in the example plugin are positioned with the wrong offset

Expected behavior:
Child flyout is aligned properly to its main flyout

Screenshots (if relevant):

1. Without layoutDebug:

01-flyout-system-with-grid-layout-no-debug.mov

2. With layoutDebug:

02-flyout-system-with-grid-layout-with-debug.mov

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)

[alexmarhaba]

Just a note here that Lens team is planning to leverage the flyout system for 9.3. I would consider this issue a blocker for releasing grid layout.