fix: viewport corruption and stale cell data from WASM page memory reuse (#138, #139)

[diegosouzapw]

Summary

Ports fixes for two related upstream issues caused by WASM internal page memory reuse:

Issue coder#139 — getViewport() corrupted data across page boundaries

• renderStateGetViewport: replaced per-row pages.pin(.active) calls with cached row pins from RenderState.row_data, matching how the native renderer reads cell data. Independent per-row resolution of the viewport top-left position could produce inconsistent page references when the viewport straddled an internal page boundary.
• terminal_new_with_config: convert scrollback_limit from line count to bytes using page layout calculation. Terminal.init expects max_scrollback in bytes, not lines — passing line counts directly resulted in a much smaller scrollback buffer, making the page-boundary condition occur far more frequently.

Issue coder#138 — Stale cell data visible on new rows after scrolling

• cursorDownScroll in Screen.zig: make row clearing unconditional. The previous if (bg_color != .none) guard skipped clearing when cursor style was default (after ESC[0m reset), leaving stale cell data from previously erased rows visible on new empty lines.

Test plan

• bun test — 397 pass, 0 fail (13 test files)
• bun run typecheck — no errors
• bun run build:wasm — clean build (patch applies, Zig 0.15.2)
• iris-repro-final.test.ts — scrollback lengths monotonically increasing, no drops
• iris-repro-fix-verify.test.ts — scrollback=10000 has 0 drops with bytes conversion
• viewport-corruption.test.ts — consistent viewport data after escape-heavy writes
• viewport-row-merge.test.ts — no row-merge corruption at cols=120/130

Inspired by: coder#133
Inspired by: coder#134

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate failed

Failed conditions
23.0% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud

[gemini-code-assist]

Code Review

This pull request addresses a WASM viewport and ring-buffer corruption issue by ensuring that the scrollback_limit is correctly converted from lines to bytes and that new row cells are cleared during scrolling to prevent stale data visibility. The Zig C API wrapper was also updated to use cached row pins in renderStateGetViewport for better consistency with the native renderer. Multiple test files were introduced to reproduce the reported bugs and verify the stability of the fixes. A critical feedback point was raised concerning a potential integer overflow in the scrollback byte calculation on 32-bit WASM, with a suggestion to use saturating multiplication to ensure safety.

[gemini-code-assist]

The calculation of scrollback_limit in bytes can overflow the usize type (which is 32-bit on WASM) if a large scrollback_limit is provided in the configuration. For example, with a large line count and wide terminal columns, the resulting byte count could exceed 4GB. It is safer to use saturating multiplication to handle this case.

    else blk: {
        // Convert lines to bytes: each page holds cap.rows rows in total_size bytes
        const cap = pagepkg.std_capacity.adjust(.{ .cols = @intCast(cols) }) catch
            break :blk std.math.mulSaturating(usize, scrollback_lines, 1024);
        const page_size = Page.layout(cap).total_size;
        const bytes_per_line = page_size / cap.rows;
        break :blk std.math.mulSaturating(usize, scrollback_lines, bytes_per_line);
    };