Fix sticky pager jumps (#1825)

* Defer showing pager content until its header settles

* Introduce the concept of headerOnlyHeight

* Keep headerOnlyHeight in state, make headerHeight derived

* Hide content until *both* header (only) and tabbar are measured

* Hide tabbar to read its layout earlier

* Give consistent keys to pages

src/view/com/pager/TabBar.tsx

@@ -14,7 +14,6 @@ export interface TabBarProps {
   indicatorColor?: string
   onSelect?: (index: number) => void
   onPressSelected?: (index: number) => void
-  onLayout?: (evt: LayoutChangeEvent) => void
 }
 
 export function TabBar({
@@ -24,7 +23,6 @@ export function TabBar({
   indicatorColor,
   onSelect,
   onPressSelected,
-  onLayout,
 }: TabBarProps) {
   const pal = usePalette('default')
   const scrollElRef = useRef<ScrollView>(null)
@@ -68,7 +66,7 @@ export function TabBar({
   const styles = isDesktop || isTablet ? desktopStyles : mobileStyles
 
   return (
-    <View testID={testID} style={[pal.view, styles.outer]} onLayout={onLayout}>
+    <View testID={testID} style={[pal.view, styles.outer]}>
       <DraggableScrollView
         horizontal={true}
         showsHorizontalScrollIndicator={false}