Sync Box mouse wheel scroll with Panel/ScrollBar impl; fix InputBox cursor stuck as IDC_IBEAM on Panel mouse-leave

[Copilot]

• Study recent PRs (Add vertical ScrollBar to Panel with smooth scrolling and Layout-integrated offset #60, Panel: clip out-of-bounds children and track ongoing animations via m_drawing #61, Fix scrollbar: replace percentage-based wheel scroll with fixed pixel distance #63) for Panel scroll implementation details
• Fix 1: Add smooth mouse wheel scroll to Box (without scrollbar), mirroring Panel/ScrollBar implementation
• Fix 2: Fix InputBox cursor shape bug in Panel (cursor stays IDC_IBEAM when mouse leaves Panel)
• Fix 3: Fix Panel scroll blocked when InputBox (or any child widget) has focus
• Fix 4: Fix cursor incorrectly set to IDC_IBEAM by scrolled-out invisible InputBox
  • Root cause: mouse-leave dispatch used real mouse coordinates; a scrolled-out InputBox positioned outside the Panel bounds could match those coordinates, calling setCursor(IDC_IBEAM)
  • Fix: use a synthetic leaveMsg with coordinates (-99999, -99999) — guaranteed outside all children — so only the lastInside reset path triggers, never the isInside = true cursor-set path
• Build verified

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[FeJS8888]

@copilot 现在有如下问题，Panel等在处理鼠标滚动的时候，如果内部输入框有焦点，无法滚动。

[Copilot]

@copilot 现在有如下问题，Panel等在处理鼠标滚动的时候，如果内部输入框有焦点，无法滚动。

Fixed in 8e4c368.

Root cause: the scrollbar wheel-event guard was focusingWidget == this, which blocked scrolling whenever a child widget (e.g. InputBox) had focus. Changed to isin (mouse is inside the Panel) — wheel events now always reach the scrollbar as long as the pointer is within the Panel, regardless of which widget has focus.

[FeJS8888]

@copilot 现在我们在鼠标停留在Panel的某个超出范围的不可见的InputBox上方，鼠标指针会被不正确的设置。请仔细阅读逻辑，进行修复

[Copilot]

@copilot 现在我们在鼠标停留在Panel的某个超出范围的不可见的InputBox上方，鼠标指针会被不正确的设置。请仔细阅读逻辑，进行修复

Fixed in 57c6db9.

Root cause: the mouse-leave dispatch was forwarding the real mouse coordinates to all children. When a child InputBox is scrolled out of the Panel viewport, its absolute position is placed outside the Panel bounds (that's how scroll works). If the real mouse happened to be at that same out-of-panel location, InputBox::isInside returned true and incorrectly set IDC_IBEAM.

Fix: replace the real coordinates with a synthetic leaveMsg at (-99999, -99999) — guaranteed outside every widget — so children only enter the lastInside → false reset path, never the isInside = true cursor-set path.