Honor background color erase when scrolling

[JohnCampionJr]

Fixes #96

Makes terminal buffers pull the active erase attributes when scrolling exposes blank cells. Only the current background is retained; foreground and rendition flags remain at their defaults.

Covers full-width upward scrolling, reverse scrolling, recycled scrollback rows, narrowed left/right margins, and alternate-screen switching.

Tests: dotnet test src/XTerm.NET.Tests/XTerm.NET.Tests.csproj --no-restore (1913 passed)

[JohnCampionJr]

Reviewed against a clean worktree; built and ran the suite (1913 passed).

The mechanism is right. Flags live in Extended, not Bg, so copying Bg alone yields bg-only and matches xterm.js _eraseAttrData(). Buffer switching is safe -- every path goes through SetBuffer.

Hot path: touched, but free. ScrollUp is reached from the print wrap-at-bottom, LF and CSI S, and now costs one null check plus a non-inlinable call returning a 12-byte struct per scrolled line -- against ResetInPlace's Array.Fill of ~4.8 KB at 240 columns. Three things I specifically checked and can confirm are unaffected: Array.Fill has no default-attribute fast path, so recycling costs the same; the print fast path gates on InsertMode/charset/HasMultiRowSizedRuns, never on attributes; and GetTrimmedLength/HasContent look at content only, so BCE-painted blanks do not start counting as content and do not perturb reflow, trimming or selection.

The concern is scope, not correctness: this gives the terminal two different BCE rules. Details inline.

[Copilot]

🟢 Approval recommended

The change cleanly implements BCE background-only erasure via a pull-based provider, updates the relevant scroll fill paths, and includes targeted regression tests for the stated scenarios.

Pull request overview

This PR implements BCE (background color erase) correctness for scrolled-in blank cells by having TerminalBuffer pull the current erase background from InputHandler when it needs to materialize new/recycled blank lines, including margin-box scrolling and alt-buffer scenarios.

Changes:

• Added an erase-attributes provider hook on TerminalBuffer, supplied by InputHandler, to fetch the current background-only erase attributes on demand.
• Updated scroll paths (full-width scroll up/down, recycled scrollback reuse, and margin-box scrolling fill) to use the provider instead of AttributeData.Default.
• Added a dedicated test suite covering forward/reverse scrolling, recycled scrollback reuse, narrowed margins, and alternate-buffer switching.

File summaries

File	Description
src/XTerm.NET/InputHandler.cs	Supplies TerminalBuffer with a background-only erase attribute provider and reattaches it on buffer switches.
src/XTerm.NET/Buffer/TerminalBuffer.cs	Uses pulled erase attributes when creating/resetting blank cells during scroll operations (including margin-box scroll).
src/XTerm.NET.Tests/BackgroundColorEraseScrollTests.cs	Adds regression tests validating BCE background behavior across scroll paths and buffer switching.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Lite

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