[ValentinYoushkevich]

global.mocks with a name like $u (uView), $m, $c or $a is never applied: the mocks mixin does this[k] = v, and Vue's instance proxy refuses to set $x when x is an internal instance field (u holds the updated hooks). The mixin throws, so mount, shallowMount and renderToString all fail, also for <script setup> components (the mixin runs on the VTU root as well).

const Comp = defineComponent({ template: '<div>{{ $u.title }}</div>' })
mount(Comp, { global: { mocks: { $u: { title: 'mocked' } } } })
// [Vue warn]: Attempting to mutate public property "$u"...
// TypeError: 'set' on proxy: trap returned falsish for property '$u'

The mixin now defines these mocks directly on the instance context, which the proxy reads them from. Object.defineProperty is used so that the mock also overrides a real $u installed by the plugin in globalProperties (this is what Vue itself does for global properties). Other mocks still go through the proxy as before.

uview-plus installs $u as a global property and uses it from components, e.g. index.js and login/index.vue.

Repro on 2.5.1: playground

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