Fix Splitter issues (#1343)

Fix Splitter issues

* Replace code with hook useAdjustedRelativeSplitValue
* Add e2e tests for splitter

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

src/components/editor-page/splitter/hooks/use-adjusted-relative-split-value.ts

@@ -0,0 +1,33 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+import { useMemo } from 'react'
+
+/**
+ * Calculates the adjusted relative split value.
+ *
+ * @param showLeft Defines if the left split pane should be shown
+ * @param showRight Defines if the right split pane should be shown
+ * @param relativeSplitValue The relative size ratio of the split
+ * @return the limited (0% to 100%) relative split value. If only the left or right pane should be shown then the return value will be always 100 or 0
+ */
+export const useAdjustedRelativeSplitValue = (
+  showLeft: boolean,
+  showRight: boolean,
+  relativeSplitValue: number
+): number =>
+  useMemo(() => {
+    let splitValue: number
+    if (!showLeft && showRight) {
+      splitValue = 0
+    } else if (showLeft && !showRight) {
+      splitValue = 100
+    } else {
+      splitValue = relativeSplitValue
+    }
+
+    return Math.min(100, Math.max(0, splitValue))
+  }, [relativeSplitValue, showLeft, showRight])