#buildhash

protected override int BuildHash() is a PanelComponent member; a logic-only Component has no such method, so the override fails headless with CS0115. Easy to hit when you split a UI feature into a logic Component beside its PanelComponent view and copy the 'keep the tree stable' BuildHash idiom onto the logic component too.

A button that never responds, with a clean console and a correct onclick handler, can be a BuildHash rebuild eating the click. If you write a focus-cursor field from onmouseover and that field participates in the panel's BuildHash, the mouse arriving over a row rebuilds the panel tree on the spot. The rebuild destroys and recreates the element under the cursor, and a press and release that straddles the rebuild is swallowed by the engine, so no onclick fires. Focus is a keyboard and gamepad concept, so never write it from a hover handler. Give the mouse its own affordance from CSS :hover, which needs no rebuild, and keep the keyboard focus index as a separate field if the panel needs both.