macOS Sequoia Betas Continue UI Tweaks

Deeper modernizations in the FileProvider framework are a core component of Sequoia's evolution, continuing a transition that began in macOS Sonoma. This shift moves cloud file synchronization, including iCloud Drive, from older kernel-level APIs to a more modern, user-space extension model. The goal is to unify how all cloud storage providers integrate with macOS, providing a more stable and secure foundation. This architectural change is likely the root cause of visual glitches like generic disk icons for cloud-synced and external drives. As the system transitions to the FileProvider API for managing file access and metadata, long-standing bugs related to Finder and File Provider extensions can be exposed, often manifesting as incorrect icon displays during the beta period. For developers, a significant update comes to concurrency with SwiftUI. Types conforming to the `View` protocol are now isolated to the @MainActor by default. This change improves compile-time diagnostics for data-race safety issues, enforcing that UI updates are handled on the main thread and reducing a common source of bugs in multi-threaded applications. Under the hood, Sequoia introduces a notable platform change for M3/A16 and later Apple Silicon chips. The timekeeping registers `CNTFRQ_EL0` and `CNTVCT_EL0` now report at 1 GHz instead of the previous 24 MHz. While apps using standard `mach_absolute_time()` are unaffected, this is a critical detail for any developer working with custom timekeeping or performance-sensitive, low-level code. Early Sequoia betas also revealed significant changes to the networking stack. The initial 15.0 release caused widespread issues with VPN clients, firewalls, and other third-party security software. These problems indicate substantial alterations to network and endpoint security frameworks, requiring developers of such tools to adapt to new system behaviors and APIs. On the AppKit side, developers will find deprecations affecting `NSToolbar`. The `allowsDuplicatesInToolbar` property is now deprecated, and an exception will be thrown if duplicate items are added. A new `allowsDisplayModeCustomization` property has been introduced, giving developers more granular control over whether users can change a toolbar's appearance. The FileProvider framework itself has gained new capabilities, including an API to allow cloud services to cache files on external storage. While not yet enabled for iCloud Drive, this points to future flexibility in managing local storage, a key consideration for users with large cloud-based file sets and smaller internal SSDs. Sequoia also introduces a "Keep Downloaded" option for iCloud Drive, a feature developers can observe in the updated FileProvider behavior. This allows users to "pin" specific files, ensuring they remain available locally even when "Optimize Mac Storage" is enabled, providing more direct control over file eviction policies.