[FEATURE]: Allow expanding and scrolling through background shell output (Web/Desktop) #62083

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Describe the enhancement you want to request

Problem

This feature request concerns the shared OpenCode v2 App frontend (packages/app), used by both the browser-based Web UI and Desktop application, rather than the terminal-based TUI.

The current background-shell activity panel introduced in #59496 displays only the latest 14 lines of output. The request is to provide expandable, scrollable access to the retained output in both Web and Desktop, ideally reusing the existing shell-output functionality rather than introducing a separate implementation.

For long-running commands, such as benchmarks, builds, test suites, and diagnostic scripts, this makes it difficult to inspect output generated earlier in the execution.

The current behavior is particularly problematic when:

• A command produces hundreds or thousands of lines of output.
• Multiple commands or tests run sequentially within the same shell.
• Important errors or diagnostic information scroll out of the visible output window.
• Users need to monitor progress and examine earlier results without interrupting the process.

Proposed solution

Enhance the background shell output viewer with the following functionality:

1. Scrollable output history: Allow users to scroll backward through all captured shell output rather than displaying only the most recent lines.
2. Expand/maximize control: Provide an option to expand the popup into a larger panel or full-height terminal-style viewer.
3. Resizable panel: Allow users to adjust the viewer's height and width.
4. Auto-scroll behavior: Automatically follow new output while the viewer is at the bottom, but preserve the scroll position when the user scrolls backward.
5. Copy output: Provide a convenient way to copy the complete captured output.

Ideally, the existing compact popup would remain the default, with an expand button for users who need additional visibility.

Alternative solutions

At minimum, adding a scrollable history and an expand button would substantially improve usability without requiring a complete terminal implementation.

An optional configurable output retention limit could help prevent excessive memory usage from commands producing very large amounts of output.

Additional context

This behavior appears related to the background shell output viewer introduced in PR #59496:

#59496

The current implementation provides a useful live preview, but it does not adequately support inspecting or debugging longer-running processes.

A screenshot demonstrating the limited popup is available.

Expected behavior

Users should be able to inspect the complete retained output of a running or recently completed background shell without needing to redirect output to a separate log file or leave the OpenCode interface.