Reset Panel ended its undo step with end_undo_group, which closes the
open undo group of every document. An undo group open in another
document, such as an MCP batch, was closed by a reset in this one, and
inside an undo group already open in this document the reset closed
that group too.
Reset Panel now opens its own group only when the active document has
none open, and closes only that group. Inside an open undo group the
reset is part of that group's step.