Fix collapsed pre-boot options form on modern macOS

[ElSteverino]

Problem

On recent macOS releases (observed on macOS 27.0 beta, 26A5388g), the options panel on a VM window's pre-boot screen collapses to its 100 pt minimum height. Only the first two rows are visible in a tiny scroll area, which hides the remaining rows — including the "Virtual Machine Settings…" button — well enough that it looks like post-install configuration doesn't exist.

Cause

SelfSizingGroupedForm sizes the grouped Form by introspecting the backing NSScrollView's documentView frame via SwiftUIIntrospect. On modern macOS the grouped form is no longer backed by an introspectable NSScrollView, so the measurement closure never fires, contentHeight stays 0, and the frame falls back to minHeight.

Fix

Use the native onScrollGeometryChange API (macOS 15+) to observe the form's scroll content height, keeping the introspection path only for macOS 13/14 (deployment target is 14.0). Same guards and behavior as before, including the VBDisableSelfSizingGroupedForm defaults escape hatch.

Testing

• Built and ran on macOS 27.0 beta with Xcode 27 beta: the pre-boot panel now fits all rows (saved-state picker, recovery/DFU toggles, capture-keys toggle, Settings button) with no inner scrolling.
• VBDisableSelfSizingGroupedForm still pins the form to minHeight when set.

🤖 Generated with Claude Code

[insidegui]

@ElSteverino have you managed to reproduce this with a recent build on main? This had been fixed in another branch that's recently been merged into main...

[ElSteverino]

Confirmed — you're right. My original repro was on the 2.1 release (build 325) on macOS 27.0 beta (26A5388g), which predates the June swiftui-introspect update. I've since built current main (74b4119) and the panel sizes correctly there, and the 2.2 beta from the update channel looks good as well. Closing — sorry for the noise, and thanks for the quick look (and for the @published fix in #727).