Bug - Popper - flash of incorrectly positioned popper on open

Describe the problem
Popper elements (dropdowns, select, etc.) occasionally flash at the top-left of the screen before appearing in their correct position.

Explanation
When a popper opens, there's a race condition between:

1. The popper element being rendered (initially with opacity: 0)
2. Popper.js calculating and applying the correct position
3. The opacity transitioning to 1

The opacity transitions to 1 before Popper.js finished calculating and applying the position transform.

How do you reproduce the problem?

4. Go to the PatternFly dropdown component page.
5. Open and close the dropdowns back to back.

Screenshots

• PF website

https://github.com/user-attachments/assets/0a076fc4-ead1-4d6a-a497-1f72ee3e28f5

• RH website

https://github.com/user-attachments/assets/4ea2b102-d9c9-4c76-8000-ef3631c6e3ab

What is your environment?

• OS: macOS Tahoe 26.3

• Browser: Safari 26.3

• CPU: M3

• Memory: 24 GB

Jira Issue: PF-3668

Start with the PatternFly dropdown component page and the Popper elements involved in opening dropdowns, then reproduce the flash by opening and closing dropdowns repeatedly in Safari. Trace the opening sequence around rendering, Popper.js positioning, and opacity; the issue is done when the popper never appears at the top-left before reaching its correct position.