The tool's default key is "+", and `shortcuts::parse` split a bare "+" as a
separator with nothing on either side: the key part came out empty, so the
shortcut didn't parse, `tool_for_key` found no tool for a typed "+" and the
key did nothing. "-" (Delete Anchor Point) has no separator to trip on, which
is why it worked. A bare "+" is now the key itself, the "=" key, as "Cmd++"
already was, so "+" typed with Shift on a US layout, by a layout's own "+"
key or on the numpad selects the tool.