gpui: Fix new window cascade positioning (#44358)

Closes [#44354](https://github.com/zed-industries/zed/discussions/44354)

Release Notes:
- Fixed new windows to properly cascade from the active window instead
of opening at the exact same position
d7b99a5b12Jake Go committed on 12/8/2025, 8:11:58 AM· committed by GitHubparent7691cf3
1 file changedLine totals unavailable