Loop Trail.

Connect neighbouring square centres into one closed loop. Visit every open square exactly once; avoid walls and islands. No branches, dead ends or smaller separate loops. Drag between cells to draw; drag back to undo. Dragging keeps existing paths. Tap a path to remove it. Tap between cells to mark or clear a cross. Arrows select a link; Enter cycles path/cross/clear; X marks a cross. Crosses are optional notes.