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The interface has been removed since IBehavior can do exactly the same. MarkupComponentBorder has been migrated, which means you can associated markup to the behavior. Markup which will surround the behavior's component.
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Component.getStyle() implementation changed
getStyle() used to return Component.getVariation() + "_" + style. That eventually caused issue such as mentioned in WICKET-2298.
We changed that to getStyle() now returning style only which as a consequence required us to change all resource related APIs to add variation. To migrate your code you maight either provide component.getVariation() or null where not relevant.