Expected: every picker on this page opens over, or attached to, the control that was clicked — never at the top-left of the window, never at the position of the previous control. A shell that opens pickers as sheets ignores the anchor; then the sheet belongs to this window and the page underneath does not move.
The engine reports the control's border box in viewport CSS px, the same space mouse clicks arrive in. Zoom the page (Cmd/Ctrl +) and try again: the anchor must follow the zoomed control.
Four controls pinned to the window corners:
Expected: scroll the box until the control is visible, then click it. The picker anchors to where the control is now, not to where it was before scrolling.
Expected: the control is drawn one and a half times its size; the picker anchors to the drawn box, not the untransformed one.
Expected: after scrolling the page itself, the control at the bottom still gets its picker where it is on screen.