# Objective

Fixes #58970.

With `terminal.copy_on_select` enabled, a left click that isn't a
deliberate selection can still overwrite the system clipboard. It
happens most often on the window-focusing click (clicking into the
terminal from another app), where the pointer jitters by a pixel or two
during the click. A perfectly motionless click is already safe (it
produces a zero-length selection and the copy is skipped), but the drag
handler treats *any* movement as the start of a selection, so a 1px
jitter creates a stray selection that `copy_on_select` then copies,
clobbering whatever was on the clipboard.

## Solution

Apply a small drag-distance threshold before a left click begins a
selection, matching the threshold gpui's `div` element already uses
(`DRAG_THRESHOLD = 2.0`). `mouse_down` records the press position, and
`mouse_drag` ignores movement below the threshold until a selection has
actually started. Once a selection is in progress it keeps extending as
before. This is the same click-vs-drag handling other terminals use
(kitty's `drag_threshold`, iTerm2).

## Testing

Added two unit tests in `crates/terminal/src/terminal.rs`:

- `test_terminal_click_jitter_does_not_start_selection` — a 1px drag
after mouse-down enqueues no `UpdateSelection` and leaves the selection
phase `Ended`.
- `test_terminal_deliberate_drag_starts_selection` — a drag well past
the threshold still starts a selection.

The first test fails on `main` (the jitter starts a selection) and
passes with this change. The rest of the `terminal` suite is green and
`script/clippy -p terminal` is clean.

I tested on Linux. The fix lives in the shared `Terminal` mouse
handling, so it's platform-independent, but I haven't manually
reproduced the macOS focusing-click scenario from the report on a Mac.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

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Release Notes:

- Fixed `terminal.copy_on_select` overwriting the clipboard when a click
jittered without an intentional drag.