Function showLoadingPanel

showLoadingPanel(
element: HTMLElement | null | undefined,
label?: string,
): Promise<void> | null
Arm a driven (AnimatedLoadingPanelProps.autoStart false) panel's fade-in, resolving once the browser has COMMITTED it. Only then may the caller seize the main thread: a class change reaches the compositor through a rendering frame, and a blocked thread never yields one, so arm-and-go would animate nothing for exactly the work worth announcing.

Whether anything is ever SEEN is the panel's transition-delay (1.2s by default), which is CSS rather than a timer for the same reason — a setTimeout cannot fire while the thread is blocked, so a JS-scheduled reveal would appear for every operation EXCEPT the long ones. Work that finishes inside the delay disarms before a pixel changes.
Parameters
- element: HTMLElement | null | undefined
  The panel's root.
- Optionallabel: string
  Replaces the panel's text (for a panel rendered with a string or absent label). Set directly rather than through a render, which could not land before blocking work.
Returns Promise<void> | null
A promise that settles once the fade is committed, or null when there is nothing to wait for — no element, or already armed (a second concurrent operation rides the first).