TUI: auto-copy highlighted text on mouse selection

[yxlyx]

Summary

Add an option for the TUI to automatically copy highlighted/selected text to the system clipboard when the user finishes a mouse drag selection.

Problem

Currently users have to select text in the TUI and then press Ctrl+C to copy it. This adds friction for a common workflow: quickly highlighting output, letting go of the mouse, and pasting it elsewhere.

Proposed behavior

• When a user drags to highlight text in the TUI, copy the selected text to the clipboard automatically when the mouse button is released.
• After copying, show a small non-blocking confirmation popup/toast at the bottom of the TUI saying: Copied to clipboard.
• The popup should disappear automatically after a short delay.

UX notes

• This should not interrupt keyboard input or require dismissal.
• Consider making the behavior configurable if automatic copy could surprise users.
• If clipboard access fails, show a brief error toast instead of silently failing.

Acceptance criteria

• Drag-selecting text in the TUI copies the selected text to the system clipboard on mouse release.
• A bottom toast/popup appears with Copied to clipboard after successful copy.
• The toast auto-dismisses after a short delay.
• Existing Ctrl+C copy behavior continues to work.
• Empty selections do not trigger clipboard writes or a toast.

[yxlyx]

Reopening: reported still broken — highlighting/selecting text in the TUI does not auto-copy as expected.

[justrach]

Partial progress in v0.0.174 (f783cbc): the new graff repl now auto-copies a mouse drag-selection to the clipboard via OSC52 on button release.

Leaving this open because it's not the full ask yet:

• it's implemented in the new graff repl, not the main harness TUI (per our "TUI harness first" rule these two shouldn't diverge);
• there's no Copied to clipboard toast/confirmation yet.

Will close once the main-TUI parity + the confirmation toast land.

Release: https://github.com/justrach/codegraff/releases/tag/v0.0.174

[justrach]

Both halves shipped.

GUI terminal pane — d9db13a, released in v0.0.167.

graff repl — mouse .release triggers copySelection (src/repl.zig:358-364); selectionText drops empty/whitespace-only selections (src/repl_model_render.zig:173); the copy goes out via OSC52 and raises a toast reading ✓ Copied to clipboard / ⚠ Clipboard copy failed (src/repl_model_render.zig:198-206, :116), auto-dismissing after TOAST_MS=1500. Pinned by two tests tagged (#85) at src/repl.zig:428 and :443. Commit b1ad22d, released in v0.0.198.

One deliberate scope note: the default readline mode never grabs the mouse (src/agent_stream.zig:223-228), so selection there stays native-terminal and is unchanged. The fix applies to graff repl, which does grab the mouse (src/repl_run.zig:32) — that's the mode the report was about. If you hit this in default mode, that's a different issue and worth filing separately.