Keep PivotViewer cards visible through layout transitions

[woksin]

Fixed

• PivotViewer no longer renders an empty viewport after regrouping or filtering when the new layout's first entries are off-screen. During a transition, cards that intersect the viewport are drawn from their target, previous, or current position. Cards that move off-screen are released when the animation ends, and a released card is not recreated from its old position while you scroll. Refs Clicking around in pivot viewer sometimes crashes it #236; the intermittent crash reported there has not been reproduced, so that issue stays open.

[woksin]

Superseded by #345, which combines this change with the other ready fixes into one PR and one release, so CI runs once.