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

### 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)

Start by reproducing mouse selection with the scrollbar enabled, then compare clipboard output with the scrollbar disabled. Trace the terminal selection and scrollbar rendering paths; the fix is done when selecting arbitrary multi-line output produces clean text without padding or the scrollbar glyph, while the scrollbar remains usable.

索引モデルが issue の本文から書いたものです。