Terminal mouse-selection copy is corrupted by the scrollbar column (cannot cleanly copy output)

[TWiStErRob]

Describe the bug

The conversation scrollbar renders a ┃ (U+2503) glyph in the rightmost column of the timeline. When I select a response with the mouse to copy it to the system clipboard, the selection includes that column — so every copied line ends with padding spaces and a trailing ┃. This makes it effectively impossible to copy clean output (code, commands, paths) out of the CLI.

This regressed with the introduction of the always-on scrollbar; copying worked fine for months before that.

Note: this is distinct from #3501, which is about the scrollbar causing line misalignment / reflow. This report is specifically about the copy/paste UX — the pasted output is unusable.

Affected version

1.0.65 – 1.0.68

Steps to reproduce the behavior

1. Run any prompt that produces multi-line output.
2. Drag-select the response with the mouse and copy to the system clipboard (not /copy).
3. Paste into any editor.
4. Every line has trailing spaces + a ┃ character appended.

Expected behavior

Selecting and copying output should yield clean text, with no scrollbar glyph or padding appended to each line — as it did before the scrollbar was added.

Additional context

• Workaround: /settings scrollbar off (or "scrollbar": false in ~/.copilot/settings.json) removes the track and fixes copying. But the default is on, so this affects everyone by default.
• /copy only helps for the latest response, not arbitrary or earlier output.
• OS: macOS 26.3.1, aarch64
• Shell: zsh, running in the integrated terminal of IntelliJ IDEA 2026.1 and Android Studio Panda 2 (2025.3.2)