feat: expose userScrolling for scroll-lock during output

[k3rnelpan11c]

Summary

• Make Terminal.userScrolling public so host apps can set it when the user scrolls away from the bottom
• Guard selection clearing in feedPrepare() and linefeed() with the userScrolling flag so active selections persist while new output streams in
• scrollWheel changed from public to open in MacTerminalView to allow subclass override

The userScrolling flag already exists and is checked in Terminal.scroll() (lines 5245, 5255, 5281) — these changes simply wire it up for external consumers.

Motivation

When using terminal-based AI tools (like Claude Code) that produce long streaming output, users cannot:

1. Scroll up to read earlier output without being snapped back to the bottom
2. Select text without the selection being cleared by incoming data
3. Maintain a selection while new content arrives

These 3 minimal changes (~6 lines) fix all three issues by leveraging the existing userScrolling infrastructure.

Test plan

• Verify default behavior unchanged (userScrolling defaults to false)
• Set terminal.userScrolling = true and verify scroll position is preserved during feed()
• Set terminal.userScrolling = true and verify text selection persists through feed()
• Verify scrollWheel can be overridden in subclass

🤖 Generated with Claude Code

[migueldeicaza]

Did you see the comments on #468?

[k3rnelpan11c]

Thanks for the pointer to #468! I've reviewed your concerns there and here's how this PR addresses them:

1. yDisp synchronization — In our approach, userScrolling is set by the host app (Flock) from within the scrollWheel override, right after super.scrollWheel() has already updated yDisp. So the flag and yDisp stay in sync — we're not toggling the flag independently of scroll state.

2. Buffer switching — Good point. When the alternate screen buffer activates (vim, less, etc.), canScroll returns false and scrollPosition returns 0, so the host app's scrollWheel override naturally sets userScrolling = false. But we should verify this edge case more thoroughly.

3. Delegate semantics — Unlike #468, we don't hijack the delegate at all. The only change is making userScrolling public so the host app can set it. The existing feedPrepare() and linefeed() selection-clearing logic just gets an additional guard (&& !terminal.userScrolling). No delegate behavior changes.

4. Scroll threshold — scrollWheel fires on any scroll input, so there's no minimum threshold. The check is scrollPosition >= 0.999 to detect "at bottom".

5. Keyboard reset — Addressed: the host app resets userScrolling = false in the send() override when the user types.

The changes are minimal (~6 lines in SwiftTerm) and all the coordination logic lives in the host app. Would you prefer we address anything else?

[migueldeicaza]

Oh, I had not seen this update, will review.

[migueldeicaza]

Would you mind testing this branch, and rebasing the above patch on this? prevent-scroll-while-dragging