With the accesskey assigned and the current tab flow, tabindex="1" seems like a relict of the past and also hindering “natural” keyboard flow.
tabindex="1" has originally been introduced to Vector as part of T48512 in https://gerrit.wikimedia.org/r/#/q/I87149d7e15931f02ab700164e9e1d3d707b5e6a5.
Current keyboard focus order seems completely confusing to me.
The general rule of thumb is
Avoid using tabindex values greater than 0. Doing so makes it difficult for people who rely on assistive technology to navigate and operate page content. Instead, write the document with the elements in a logical sequence.
This seems to apply here as well.