Faster on pages that change as you use them - plus a tab-hover fix

Two speed improvements to the built-in engine and one polish fix. When a page updates part of itself as you interact - a menu opening, a list filtering, a form reacting - the engine now redoes the styling for just the part that changed instead of the whole page, so busy sites feel quicker. Pages that lean on script to measure their own layout are much faster too. And hovering a tab no longer briefly blanks the page you were reading.