Preserve rows below partial scroll regions

[jchristn]

Summary

• Treat only full-screen scroll regions as eligible for scrollback promotion
• Keep top-anchored partial scroll regions in-place so rows below the bottom margin are preserved
• Use active-buffer coordinates for IL / CSI Ps L and DL / CSI Ps M when normal-buffer scrollback exists
• Ignore DL / CSI Ps M when the cursor is outside the scroll region, matching IL
• Add regression tests covering reserved bottom prompt/status rows

Why

Prompt-oriented terminal applications can reserve a bottom input/status row while scrolling output above it. A top-anchored partial scroll region must not promote the reserved row into scrollback.

The same visual artifact can occur after normal-buffer scrollback exists if insert/delete-line operations splice using viewport-relative ScrollBottom rather than YBase + ScrollBottom. In that case old scrollback rows can be edited instead of the active screen, allowing prompt/status content such as > or > to reappear higher in the viewport.

DL also needs the same scroll-region guard as IL; delete-line operations outside the active scroll region should be ignored so reserved prompt/status rows are not shifted.

Tests

• dotnet test src\XTerm.NET.slnx

[Copilot]

Pull request overview

This PR adjusts scroll/scrollback and insert/delete-line behavior to correctly preserve reserved rows (e.g., bottom prompt/status lines) when applications use top-anchored partial scroll regions, and to ensure IL/DL operations target the active screen when normal-buffer scrollback exists.

Changes:

• Restrict scrollback promotion to full-screen scroll regions, keeping top-anchored partial regions scrolling in-place.
• Use active-buffer (absolute) coordinates for IL (CSI Ps L) and DL (CSI Ps M) when YBase is non-zero, and ignore DL outside the scroll region (matching IL).
• Add regression tests covering partial scroll regions and IL/DL behavior with scrollback and reserved prompt rows.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/XTerm.NET/InputHandler.cs	Fixes IL/DL splice indices to use active-buffer coordinates and adds a scroll-region guard for DL.
src/XTerm.NET/Buffer/TerminalBuffer.cs	Limits scrollback promotion to full-screen scroll regions so reserved rows below partial regions aren’t promoted.
src/XTerm.NET.Tests/Buffer/BufferTests.cs	Adds regression tests for partial scroll regions and IL/DL behavior when scrollback exists.

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