Add VirtualFlow.show(cell, region) method.
Print errors encountered during event processing.
VirtualFlow: Fix off-by-one error.
Fix Up/Down arrow navigation when the caret is not in the viewport.
Remove commented out and unused code.
VirtualFlow: Use Bounds instead of Rectangle as cell region specification.
VirtualFlow: ensure instant cell layout when placing cells.

So that cells can immediately be used for measuring purposes.
Improve caret following.