fix: account for ListHeaderComponent length when calculating offset… (

#17415)

Summary:
… in VirtualizedList - fixes #16612

Issue is detailed in #16612

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I need `onViewableItemsChanged` to account for the length of any ListHeaderComponents

I couldn't find any tests that currently cover the use-case of a VirtualizedList + ListComponent + scroll with onViewableItemsChanged, so I have not added any tests - all previous tests pass however.

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[GENERAL][BUGFIX][VirtualizedList] - account for `ListHeaderComponent` length
Closes #17415

Differential Revision: D8683555

Pulled By: hramos

fbshipit-source-id: 05df7b79c16e3c07c12468e782f3c4b0bdce7403

Libraries/Lists/VirtualizedList.js

@@ -1217,7 +1217,9 @@ class VirtualizedList extends React.PureComponent<Props, State> {
   }
 
   _selectOffset(metrics: $ReadOnly<{x: number, y: number}>): number {
-    return !this.props.horizontal ? metrics.y : metrics.x;
+    return (
+      (!this.props.horizontal ? metrics.y : metrics.x) - this._headerLength
+    );
   }
 
   _maybeCallOnEndReached() {