fix: Alters main menu selection to not cause full re-render

[johncowen]

Back in #2662 we moved the main navigation in App.vue to use route.child() for selection.

I noticed that this is causing a full page re-render whenever the route is changed. You can notice this because our Github button in the header flickers on every single navigation (caused by the re-render). When navigating between tabbed sub-pages you shouldn't see a flicker (but you will do when navigation between Home, Zones, Meshes).

Strangely route.child() doesn't cause full page re-renders in other areas, in fact it was used/made specifically to avoid full page re-renders.

For the moment I've added a fix straight into App.vue that avoids the issue. But I would guess at some point I will come back to try and ensure that route.child() or some iteration of it works consistently across all usages.

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[johncowen]

Wanted to note that this PR is good and its a good fix for the regression explained in the PR desc, but!

You can notice this because our Github button in the header flickers on every single navigation (caused by the re-render).

I noticed that only the Github button has its DOM elements re-rendered. Whilst this was a good indication that we were re-calculating the rendering of things that we shouldn't. The DOM for the GH button shouldn't get re-rendered at all following this recalculation. I took a look at the vue-github-buttons package that we use for this and I think its the way that the vie implementation is handled. The vue implementation further uses another github-buttons package, so theoretically we could replace the Vue implementation with our own that doesn't have a needless re-render, but still depend on github-buttons, which would be a small but worthwhile technical improvement.

I'll probably PR something to cover ^ at some point soon.