fix(core): fix _hasManualScroll lifecycle for correct sticky scroll re-engagement

[Matteo-DelliRocioli]

Summary

Two fixes to make _hasManualScroll behave correctly in ScrollBoxRenderable:

1. Guard the else branch in recalculateBarProps() with !this._hasManualScroll, making it consistent with the stickyStart branch which already has this guard.

2. Remove redundant _hasManualScroll = true sets in onMouseEvent and handleKeyPress that unconditionally override the correct state already managed by the scrollTop/scrollLeft setters and updateStickyState().

Context

This is a follow-up to the work done in #531 and #722, which addressed two earlier aspects of the same underlying problem:

• ScrollBox _hasManualScroll is never reset, breaking stickyScroll behavior #530 / fix(scrollbox): reset _hasManualScroll when user scrolls back to sticky position #531 — _hasManualScroll was never reset, permanently breaking sticky scroll after user interaction. Fixed by clearing the flag in updateStickyState() when the user scrolls back to the sticky edge.
• ScrollBox: content size changes reset _hasManualScroll, causing auto-scroll during streaming #709 / fix reset during streaming #722 — Content size changes during streaming accidentally cleared _hasManualScroll through updateStickyState() during recalculation. Fixed by adding the _isApplyingStickyScroll re-entrancy guard.

PR #722 solved the flag corruption (the flag is no longer accidentally cleared), but two problems remained:

Problem 1: Missing guard in recalculateBarProps()

The else branch force-scrolls via _stickyScrollBottom / _stickyScrollTop without checking _hasManualScroll:

// Branch 1: correctly guarded ✅
if (this._stickyStart && !this._hasManualScroll) {
    this.applyStickyStart(this._stickyStart)
// Branch 2: missing guard ❌
} else {
    if (this._stickyScrollBottom && newMaxScrollTop > 0) {
        this.scrollTop = newMaxScrollTop  // force-scrolls even if user scrolled up
    }
}

Fix: Change } else { to } else if (!this._hasManualScroll) {.

Problem 2: Redundant _hasManualScroll = true in input handlers

Both onMouseEvent (line 571) and handleKeyPress (lines 585, 591) unconditionally set _hasManualScroll = true after scroll events. This runs after updateStickyState() has already correctly cleared the flag (when the user is at the sticky edge), permanently re-setting it to true:

1. onMouseEvent → this.scrollTop += delta
2.   scrollTop setter → updateStickyState() → "at bottom" → _hasManualScroll = false ✅
3. Back in onMouseEvent:
     this._hasManualScroll = true  // ← overrides the correct state! ❌

This means scrolling to the bottom never re-engages auto-scroll, because _hasManualScroll is stuck on true.

Fix: Remove the redundant sets. The scrollTop/scrollLeft setters already handle _hasManualScroll correctly with isAtStickyPosition() checks, and the scrollbar onChange callbacks do the same. The removed code only ran in two cases:

• After actual scroll movement → already handled by the setter
• After sub-pixel accumulation (no actual movement) → shouldn't mark as manual scroll anyway

Why this matters for consumers with stickyStart set

Any consumer using stickyStart="bottom" (e.g., OpenCode's session view) is affected:

• Normal auto-scroll (user hasn't scrolled up): _stickyStart is truthy, _hasManualScroll is false → true && true → branch 1 runs → applyStickyStart("bottom") → correctly auto-scrolls. Branch 2 is never reached.

• User scrolls up during streaming: _stickyStart is still truthy, but _hasManualScroll is now true → true && false → branch 1 fails → falls to branch 2. Without the guard, branch 2 force-scrolls back to bottom.

So for any consumer with stickyStart set, the only way to reach branch 2 is when _hasManualScroll is true — exactly the case where force-scrolling should be blocked.

How it works end-to-end

• User scrolls up during streaming: _hasManualScroll = true → guard blocks force-scrolling → viewport stays put
• User scrolls back to bottom: scrollTop setter sees isAtStickyPosition() === true → doesn't set _hasManualScroll; updateStickyState() clears it → _hasManualScroll = false → auto-scroll resumes on next content resize
• On submit / session change: scrollTo resets position → _hasManualScroll clears → sticky scroll re-engages

Test plan

• Verify auto-scroll to bottom works normally when not manually scrolling
• Scroll up during streaming content growth → viewport should stay at user's position
• Scroll back to bottom → auto-scroll should re-engage
• Test with stickyStart="bottom" and without stickyStart set
• Verify horizontal sticky scroll behaves consistently
• Keyboard scrolling (arrow keys / page up/down) follows same behavior

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commit: 3c4fb75