Upon selecting items in "multiple" GlListbox unexpectedly closes

Description

When setting up GlListbox with "multiple='true'", selecting multiple items should keep the list open to make multiple selections, however GlListbox unexpectedly closes in some cases.

I haven't tested this fully, but I suspect it has to do with how the options are generated: I re-compute options when selections occur which leads to a renewed options object.

See the draft implementation MR for more details: gitlab!101608 (closed)