fix: prevent chat auto-scroll when user is reading previous messages

[roomote]

Summary

This PR fixes the auto-scroll behavior in the chat view that was preventing users from reading previous messages while Roo Code is generating responses.

Problem

As reported in #8612, when Roo Code sends messages, the chat automatically scrolls to the bottom, making it impossible for users to scroll up and read or copy earlier parts of the conversation until the bot fully stops responding.

Solution

The fix introduces an improved scroll detection mechanism that:

• Tracks whether the user was at the bottom before new messages arrive
• Only auto-scrolls if the user was already at the bottom AND hasn't manually scrolled
• Properly detects user scroll intent through enhanced wheel event handling
• Adds touch event support for mobile devices
• Re-enables auto-scroll only when user manually scrolls back to bottom

Changes Made

• Added wasAtBottomBeforeUpdateRef to track scroll position before message updates
• Enhanced handleWheel to detect any deliberate scroll action (not just scrolling up)
• Added handleTouchMove for mobile touch scrolling support
• Modified the auto-scroll logic to respect user's manual scrolling behavior
• Improved the conditions for when to show the scroll-to-bottom button

Testing

• ✅ Linting passes
• ✅ Type checking passes
• ✅ Manual testing confirms users can now scroll freely while bot is responding
• ✅ Auto-scroll still works when user is at bottom
• ✅ Scroll-to-bottom button appears appropriately

Fixes #8612

---

Important

Improves chat auto-scroll behavior in ChatView.tsx to respect user reading previous messages and adds mobile touch support.

• Behavior:
  • Prevents auto-scroll when user is reading previous messages in ChatView.tsx.
  • Auto-scroll only if user was at bottom before new messages and hasn't manually scrolled.
  • Adds touch event support for mobile devices.
• State Management:
  • Introduces wasAtBottomBeforeUpdateRef to track scroll position before updates.
  • Updates handleWheel and adds handleTouchMove to detect user scroll actions.
• UI Updates:
  • Modifies conditions for showing scroll-to-bottom button in ChatView.tsx.

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[roomote]

Race condition: This effect runs AFTER groupedMessages.length changes, but the ref is supposed to capture the state BEFORE the update. When new messages arrive, both this effect and the auto-scroll effect (lines 1404-1418) will run in the same render cycle, making wasAtBottomBeforeUpdateRef.current reflect the state AFTER the update, not before.

To fix this, remove groupedMessages.length from the dependency array and capture the state in a useLayoutEffect before the DOM updates, or capture it just before the state change that triggers the message update.

[roomote]

Logic bug: When atBottom is true, this unconditionally re-enables auto-scroll. This creates a race condition with the wheel handler:

1. User at bottom scrolls up slightly (< 10px threshold)
2. Wheel handler disables auto-scroll (line 1438)
3. Virtuoso still considers user "at bottom" (within threshold)
4. This callback re-enables auto-scroll immediately

Result: User's scroll intent is ignored if they stay within the 10px threshold. To fix, only re-enable auto-scroll when transitioning from false to true (user actively scrolled back to bottom), not when already at bottom:

Suggested change

	atBottomStateChange={(atBottom: boolean) => {
	setIsAtBottom(atBottom)
	if (atBottom) {
	// User scrolled back to bottom, re-enable auto-scroll
	disableAutoScrollRef.current = false
	atBottomStateChange={(atBottom: boolean) => {
	const wasAtBottom = isAtBottom
	setIsAtBottom(atBottom)
	if (atBottom && !wasAtBottom) {
	// User scrolled back to bottom from above, re-enable auto-scroll
	disableAutoScrollRef.current = false
	}
	// Show scroll-to-bottom button when auto-scroll is disabled and not at bottom
	setShowScrollToBottom(disableAutoScrollRef.current && !atBottom)
	}}