The drag-drop / file-drop callback selected the dropped assemblies with a
raw SelectedItems.Clear() + Add() loop, which flashes a transient empty
selection. The model has SelectNodes precisely to avoid that -- it brackets
the swap so the selection-changed fan-out fires once, with the final set,
never empty. A transient empty poisons the grid-sync deferred guard, after
which the tree stops following tab activation. Route the callback through
SelectNodes; a regression test asserts no empty SelectedItem is observed
mid-drop.
Assisted-by: Claude:claude-opus-4-8:Claude Code
Siegfried Pammer