Clicking to focus child inside ScrollablePanelControl causes scroll jump

[nickprotop]

Description

When clicking on a visible interactive control inside a scrolled-down ScrollablePanelControl (SPC), the panel jumps — often to the top. This breaks click-to-focus UX in any dialog with scrollable content.

Root Cause

FocusManager.SetFocus(spc, FocusReason.Mouse) treats SPC as an IFocusScope and delegates focus to GetInitialFocus(), which returns the first focusable child — not the one actually clicked. This triggers ScrollChildIntoView on the wrong child, causing a scroll jump. The control's own ProcessMouseEvent then correctly focuses the clicked child, but the scroll damage is already done.

The chain:

1. HandleClickFocus → FocusManager.HandleClick walks up to SPC
2. SetFocus(SPC, Mouse) sees IFocusScope → calls GetInitialFocus(false) → returns first child
3. EnterOrFocus(firstChild) hardcodes FocusReason.Keyboard, losing the Mouse reason
4. ScrollChildIntoView(firstChild) fires → scroll jumps to first child
5. SPC's ProcessMouseEvent then correctly focuses the clicked child — but too late

Expected Behavior

Clicking a visible control inside a scrolled SPC should focus that control without any scroll position change.