Restore tab selection during drag

[wieslawsoltes]

PR Summary: Issue 861 - Restore tab selection during drag

Summary

This change fixes an interaction bug where clicking a different tab and immediately starting a drag could switch the selected document/tool before the dock drag began. The selection now stays on the originally active tab when a drag threshold is reached or when the pointer exits the tab strip bounds to begin a dock drag. This prevents accidental tab activation and avoids focus/preview jumps while initiating a drag operation.

Implementation details

Selection capture and restore

• The tab drag helper (ItemDragHelper) now captures the current selection on pointer press when the owner sits inside a SelectingItemsControl (e.g., TabStrip).
• When drag initiation happens, selection is restored in two scenarios:
  • Out-of-bounds drag start (pointer leaves the tab strip to start a dock drag).
  • Drag threshold met (dragging far enough to start reordering).
• Selection is restored by preferred item, then by index if the item is no longer in the collection, and is only done once per drag sequence.
• All tracking state is cleared on release, capture loss, and detach to avoid stale state.

No API surface changes

• Behavior is internal to ItemDragHelper and applies to all controls that use it:
  • DocumentTabStripItem
  • ToolTabStripItem
  • ToolPinItemControl
• No public API or XAML changes are required for consumers.

Usage / behavior notes

User-facing behavior

• Clicking a tab and immediately dragging it does not switch the active tab unless the drag ends in a reorder/drop.
• When a drag leaves the bounds (to begin dock drag), selection stays on the original tab.
• When reordering tabs within the strip, the selection still follows the dropped item as before.

Developer-facing behavior

• No configuration is needed; the fix is automatic for any control using ItemDragHelper.
• If you subclass/compose a custom tab strip and use ItemDragHelper, the same behavior applies without additional work.

Tests

New headless tests validate both restoration paths:

• DocumentTabStripSelectionRestoreTests.DragOutside_RestoresSelection
• DocumentTabStripSelectionRestoreTests.DragThreshold_RestoresSelection

These tests simulate pointer press/move sequences and verify that selection returns to the original tab when drag starts.

Fixes #861