macOS: Safari window resizes to full width with Spotify

Hello everyone,

I am experiencing a highly frustrating and persistent window management bug on macOS 27 Golden Gate (version 27.0.1).

The Problem:

I do not use my apps in native Full Screen mode (the green button). Instead, I manually adjust my application windows (like Safari) to precise, custom widths and lengths so I can arrange them on my desktop and multi-task (without using native Split View).

Recently, Safari has stopped remembering its custom dimensions. Whenever I am working in Safari and switch focus to another application—specifically Spotify—the Safari window automatically resizes and expands to the full width of the screen on its own, completely overriding the custom size I manually set. This happens instantly without clicking any resize buttons.

System Environment:

• OS: macOS 27 Golden Gate (v27.0.1)
• Hardware: Mac
• Primary Apps Involved: Safari, Spotify (desktop client)

Troubleshooting Attempted (All Have Failed):

To resolve this, I have already attempted multiple standard and advanced troubleshooting steps, none of which have stopped the behavior:

1. Window Management Toggles: I have completely turned OFF all native automatic window snapping and tiling features in settings, including "Drag windows to screen edges to tile" and "Drag windows to menu bar to fill screen".
2. Stage Manager: Stage Manager is completely disabled.
3. Application Switching Settings: In Mission Control settings, the option "When switching to an application, switch to a space with open windows for the application" has been turned OFF.
4. Double-Click Behavior: Title bar behaviors (like double-clicking to zoom/fill) have been deactivated or set to do nothing.
5. Window Zoom State: Checked the native Safari Window -> Zoom menu state to ensure it wasn't stuck in a continuous macro zoom loop.
6. Terminal Defaults Commands: Attempted to isolate and purge the window frame state via terminal plist modifiers, including forcing a custom frame autosave layout identifier and purging the window restoration cache using:
7. defaults delete com.apple.Safari NSWindowRestorationState
8. Cache & Preference Purging: Located and deleted the core Safari preference configuration files (com.apple.Safari.plist) and the structural window cache directories within ~/Library/Caches/com.apple.Safari to force a hard layout rebuild upon a full system restart.
9. Layout Memory Forcing: Attempted to use the keyboard modified close sequences (Command + Option + Q and holding Option while interacting with window boundaries) to hard-write the default bounds to the OS layout engine.

Despite all automatic window management features being entirely disabled and the caches cleared, the window system completely ignores the custom frame boundaries. The second Safari loses focus to Spotify, it balloons to full horizontal width.

Has anyone else encountered this layout persistence bug on Golden Gate 27.0.1? Is there a hidden layout configuration file or a known daemon conflict between Spotify's desktop framework and Safari's window focus loss triggers that causes this?

Any technical insight or terminal overrides to lock application boundaries permanently would be greatly appreciated.