From the proposal's open questions:

Show/hide or show/close? This proposal introduces a symmetrical show/hide method pair on the proposed popup element. dialog, however, sets a precedent for a show and close method pairing. That seemed less intuitive, but perhaps the existing pattern should be followed. Alternatively, the platform could introduce hide on dialog and consider close deprecated.

This consistency feedback came up on MicrosoftEdge/MSEdgeExplainers#455