1 files changed, 70 insertions, 0 deletions
diff --git a/web/src/js/components/helpers/VirtualScroll.js b/web/src/js/components/helpers/VirtualScroll.js
new file mode 100644
index 00000000..5d4cf796
--- /dev/null
+++ b/web/src/js/components/helpers/VirtualScroll.js
@@ -0,0 +1,70 @@
+/**
+ * Calculate virtual scroll stuffs
+ *
+ * @param {?Object} opts Options for calculation
+ *
+ * @returns {Object} result
+ *
+ * __opts__ should have following properties:
+ * - {number}         itemCount
+ * - {number}         rowHeight
+ * - {number}         viewportTop
+ * - {number}         viewportHeight
+ * - {Array<?number>} [itemHeights]
+ *
+ * __result__ have following properties:
+ * - {number} start
+ * - {number} end
+ * - {number} paddingTop
+ * - {number} paddingBottom
+ */
+export function calcVScroll(opts) {
+    if (!opts) {
+        return { start: 0, end: 0, paddingTop: 0, paddingBottom: 0 };
+    }
+
+    const { itemCount, rowHeight, viewportTop, viewportHeight, itemHeights } = opts;
+    const viewportBottom = viewportTop + viewportHeight;
+
+    let start = 0;
+    let end = 0;
+
+    let paddingTop = 0;
+    let paddingBottom = 0;
+
+    if (itemHeights) {
+
+        for (let i = 0, pos = 0; i < itemCount; i++) {
+            const height = itemHeights[i] || rowHeight;
+
+            if (pos <= viewportTop && i % 2 === 0) {
+                paddingTop = pos;
+                start = i;
+            }
+
+            if (pos <= viewportBottom) {
+                end = i + 1;
+            } else {
+                paddingBottom += height;
+            }
+
+            pos += height;
+        }
+
+    } else {
+
+        // Make sure that we start at an even row so that CSS `:nth-child(even)` is preserved
+        start = Math.max(0, Math.floor(viewportTop / rowHeight) - 1) & ~1;
+        end = Math.min(
+            itemCount,
+            start + Math.ceil(viewportHeight / rowHeight) + 1
+        );
+
+        // When a large trunk of elements is removed from the button, start may be far off the viewport.
+        // To make this issue less severe, limit the top placeholder to the total number of rows.
+        paddingTop = Math.min(start, itemCount) * rowHeight;
+        paddingBottom = Math.max(0, itemCount - end) * rowHeight;
+    }
+
+    return { start, end, paddingTop, paddingBottom };
+}