Auto-scroll support in Section Translation (T273752) is helpful to keep the sentence to translate always visible. However, currently it acts also on sentences that are completely in the viewport. When tapping on one of those sentences, the viewport moves in an unexpected way. This is illustrated in the video below:
This ticket proposes to not apply auto-scroll to sentences that are completely inside the viewport already.
Note that for sentences that are partially shown in the viewport, the auto-scroll should be kept to help showing most of them inside the viewport. The video below shows examples where this happens which should still work like this once the issue described above gets fixed: