Confirm viewport cancellation and restore source focus

[cxa]

Summary

• prompt for confirmation before canceling a viewport containing nonblank content
• treat whitespace-only viewport content as empty
• keep the viewport open when discard confirmation is declined
• guard alternate recursive-edit exit and abort paths before viewport cleanup
• restore focus to the original source input window after cancellation when it is still available
• avoid forcing window-layout changes when the source window is no longer available

Testing

• added regression coverage for confirmed and declined cancellation
• added coverage for whitespace-only and narrowed viewport content
• added full-lifecycle coverage for alternate exit, abort, and source-focus restoration
• passed all 1,271 tests with 0 unexpected failures and 13 existing skips
• passed byte compilation, checkdoc, and git diff --check

[tninja]

Thanks for the contribution—this is a thoughtful improvement to viewport cancellation safety and source-focus restoration. I left one blocking lifecycle comment and one explicitly non-blocking defense-in-depth suggestion.

[cxa]

@tninja Thanks for the review. I addressed the blocking lifecycle issue and the defense-in-depth remaps in 00f59a0, replied on both threads, and reran the full suite (1,271 tests, 0 unexpected failures). Could you take another look when convenient?

[tninja]

Thank for the updates! Will review by the end of day.

[tninja]

Thanks for addressing the feedback so thoroughly. The lifecycle guard and recursive-exit remaps close the data-loss path, and the updated coverage looks appropriate. Approved—thank you for the contribution!