fix: Move content into to frontend directory

Doing this BEFORE the merge prevents a lot of merge conflicts.

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/src/components/render-page/hooks/sync-scroll/use-scroll-to-line-mark.ts

@@ -0,0 +1,69 @@
+/*
+ * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import type { RefObject } from 'react'
+import { useCallback, useEffect, useRef } from 'react'
+import type { ScrollState } from '../../../editor-page/synced-scroll/scroll-props'
+import { findLineMarks } from '../../../editor-page/synced-scroll/utils'
+import type { LineMarkerPosition } from '../../../markdown-renderer/extensions/linemarker/types'
+
+/**
+ * Scrolls the given container to the correct {@link LineMarkerPosition}.
+ *
+ * @param scrollState The current {@link ScrollState}
+ * @param lineMarks An array of the current {@link LineMarkerPosition LineMarkerPositions}.
+ * @param contentLineCount The number of lines
+ * @param scrollContainer The container to scroll in
+ */
+export const useScrollToLineMark = (
+  scrollState: ScrollState | undefined,
+  lineMarks: LineMarkerPosition[] | undefined,
+  contentLineCount: number,
+  scrollContainer: RefObject<HTMLElement>
+): void => {
+  const lastScrollPosition = useRef<number>()
+
+  const scrollTo = useCallback(
+    (targetPosition: number): void => {
+      if (!scrollContainer.current || targetPosition === lastScrollPosition.current) {
+        return
+      }
+      lastScrollPosition.current = targetPosition
+      scrollContainer.current.scrollTo({
+        top: targetPosition
+      })
+    },
+    [scrollContainer]
+  )
+
+  useEffect(() => {
+    if (!scrollContainer.current || !lineMarks || lineMarks.length === 0 || !scrollState) {
+      return
+    }
+    if (scrollState.firstLineInView < lineMarks[0].line) {
+      scrollTo(0)
+      return
+    }
+    if (scrollState.firstLineInView > lineMarks[lineMarks.length - 1].line) {
+      scrollTo(scrollContainer.current.offsetHeight)
+      return
+    }
+    const { lastMarkBefore, firstMarkAfter } = findLineMarks(lineMarks, scrollState.firstLineInView)
+    const positionBefore = lastMarkBefore ? lastMarkBefore.position : lineMarks[0].position
+    const positionAfter = firstMarkAfter ? firstMarkAfter.position : scrollContainer.current.offsetHeight
+    const lastMarkBeforeLine = lastMarkBefore ? lastMarkBefore.line : 1
+    const firstMarkAfterLine = firstMarkAfter ? firstMarkAfter.line : contentLineCount
+    const linesBetweenMarkers = firstMarkAfterLine - lastMarkBeforeLine
+    const blockHeight = positionAfter - positionBefore
+    const lineHeight = blockHeight / linesBetweenMarkers
+    const position =
+      positionBefore +
+      (scrollState.firstLineInView - lastMarkBeforeLine) * lineHeight +
+      (scrollState.scrolledPercentage / 100) * lineHeight
+    const correctedPosition = Math.floor(position)
+    scrollTo(correctedPosition)
+  }, [contentLineCount, lineMarks, scrollContainer, scrollState, scrollTo])
+}