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