fix(theme): adjust the position of the curtain to avoid block sidebar

[CHOYSEN]

refs #1049 #1709

reproduction

https://vitepress.vuejs.org/guide/what-is-vitepress

try collapse the first item of sidebar

[CHOYSEN]

After this change, maybe the shadow(background: linear-gradient(var(--vp-c-bg), transparent 70%)) can be apply in .curtain directly instead of the before element. Did i miss anything :)

vitepress/src/client/theme-default/components/VPNavBar.vue

Lines 173 to 179 in 48f0b01

	.VPNavBar.has-sidebar .curtain::before {
	display: block;
	width: 100%;
	height: 32px;
	background: linear-gradient(var(--vp-c-bg), transparent 70%);
	content: "";
	}

[brc-dd]

/cc @kiaking

PS: if this 100% thing was intentional, then probably we can revert this and just add pointer-events none.

[kiaking]

No, it wasn't! This was planned so all good 👍