Corner grip
Resizing snaps rather than moving freely, and that is intentional. A board whose width is 2.4 icons wide has a ragged column of dead space on its right edge. Snapping to the Icon grid means a board is always exactly as wide as the icons it can hold.
The grip also snaps to the edges of the Work area and to the edges of other boards, so a row of boards lines up without manual nudging. If boards do end up misplaced — after a resolution change, for example — the Tray menu has a Fix misplaced boards command that re-tiles only the ones that are actually wrong.