Restyle hint on an element with text content causes whole documented to be reflowed

[mbrubeck]

The test case at #10499 (comment) hits a performance problem, in addition to the text layout issue that it is demonstrating.

The issue is:

1. When the node is hovered, the :hover selector on the <a> element causes style::dom::TElement::note_restyle_hint to dirty the element and its descendants when it's hovered.
2. Because it is dirty, the flow constructor tries to process the text node inside the <a> element.
3. repair_if_possible tries to repair the text node but fails because its construction result contains unscanned text fragments, which we can't repair properly (#7821).
4. Because it can't be repaired, the text node is reconstructed. The newly constructed node has REFLOW damage. compute_layout_damage propagates this damage up and down the flow tree, causing a bunch of stuff to be reflowed.

An ideal fix for this would be a way to repair scanned text fragments.