Expose lines leaving the terminal viewport

[simonefil]

Motivation

Headless terminal hosts may need to persist or process rows as they leave the active viewport. One example is maintaining an external, bounded history that survives beyond
XTerm.NET's in-memory scrollback.

The existing Scrolled, Trimmed, and BufferChanged events do not provide the affected BufferLine before it is moved, removed, or recycled. A host therefore cannot reliably
capture the final contents and attributes of that row.

Changes

This adds a LineExitedViewport event to Terminal. The event reports:

• the BufferLine leaving the viewport;
• whether it belongs to the normal or alternate buffer;
• whether it exited because of scrolling or buffer deactivation.

Scroll notifications are raised synchronously before the line can be spliced or recycled. Full-width scroll regions are supported; narrowed left/right margins do not raise the
event because they move only part of each row.

When switching buffers, the event is raised for meaningful visible rows before the previous buffer is deactivated. Trailing empty rows are omitted.

[Copilot]

Copilot review overview

🟡 Changes recommended

Downward scrolling, reset-driven deactivation, and metadata-only rows can currently lose lines without notification.

Review effort: Balanced
Findings: 3 · 1

Open (4)

• Notify Scrolled before removing rows during downward scrolling · New
• Emit BufferDeactivated when Reset switches from the alternate buffer · New
• Preserve trailing rows containing metadata or styled blank cells · New
• Document that callback Line buffers may be immediately recycled · New

What changed in this PR

Adds notifications for lines leaving the active terminal viewport.

Changes:

• Introduces LineExitedViewport and exit-reason metadata.
• Emits notifications during upward scrolling and buffer switches.
• Adds coverage for scrolling, margins, and buffer switching.

File	Description
Terminal.cs	Exposes and raises viewport-exit events.
Events/​TerminalEvents.cs	Defines event arguments.
Common/​Types.cs	Defines line-exit reasons.
Buffer/​TerminalBuffer.cs	Emits notifications during upward scrolling.
LineExitedViewportTests.cs	Tests core event behavior.

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[JohnCampionJr]

@simonefil this is a good idea; can you address the code reviews from copilot please?

[simonefil]

Thanks for the review! All four Copilot findings have now been addressed in 1ff4cec, i also added regression tests for these cases