[Bug Report] Hide on scroll doesn't remove v-main padding when scrolling using custom scroll target

[mcfarljw]

Environment

Vuetify Version: 2.4.2 Vue Version: 2.6.12 Browsers: Chrome 88.0.4324.77 OS: Mac OS 11.1.0

Steps to reproduce

1. Add a scrollable fixed height content area
2. Add an v-app-bar with hide-on-scroll and a custom scroll-target
3. Set the scroll-target to the fixed height content area

Expected Behavior

When scrolling the content gets bumped up when the app bar extension area is hidden.

Actual Behavior

When scrolling the extension area is hidden but the v-main padding is not removed so there is a gap.

Reproduction Link

https://codesandbox.io/s/vuetify-scroll-target-2f55k

Other comments

The reproduction link is a simplified example. The real world use case which caused me to run into this issue was using the v-virtual-scroll component which requires a fixed height for scrolling and also trying to take advantage of hiding the app bar.

[johnleider]

v3 reproduction: https://play.vuetifyjs.com/#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