add "auto" mode for triggerHeight which triggers if the gesture is an overscroll

examples/Auto.tsx

+import * as React from "react";
+import { PullDownContent, PullToRefresh, RefreshContent, ReleaseContent } from "../src";
+
+export interface AutoProps {
+}
+
+export interface AutoState {
+}
+
+export class Auto extends React.Component<AutoProps, AutoState> {
+    private onRefresh() {
+        return new Promise((resolve) => {
+            setTimeout(resolve, 2000);
+        });
+    }
+
+    public render() {
+        return (
+            <div style={{overflow: "scroll"}}>
+                <PullToRefresh
+                    pullDownContent={<PullDownContent />}
+                    releaseContent={<ReleaseContent />}
+                    refreshContent={<RefreshContent />}
+                    pullDownThreshold={200}
+                    onRefresh={this.onRefresh}
+                    triggerHeight="auto"
+                    backgroundColor="white"
+                >
+                    <div id="basic-container">
+                        <div id="basic-label">PullToRefresh</div>
+                    </div>
+                    <div>
+                        <h2>Scrollable content</h2>
+
+                        <div id="scrollable-content" />
+                    </div>
+                    <style>{`
+                        #basic-container {
+                            height: 50vh;
+                            display: flex;
+                            flex-direction: column;
+                            align-items: center;
+                            background: darkslategray;
+                        }
+                        #basic-label {
+                            user-select: none;
+                            margin-top: 20px;
+                            color: aliceblue;
+                            border: 1px solid aliceblue;
+                            border-radius: 6px;
+                            padding: 5px 2px;
+                        }
+                        #basic-label:hover {
+                            cursor: pointer;
+                        }
+                        #scrollable-content {
+                            height: 100vh;
+                            width: 100%;
+                            background: darkmagenta;
+                        }
+                    `}</style>
+                </PullToRefresh>
+            </div>
+        );
+    }
+}

src/components/PullToRefresh.tsx

 import * as React from "react";
+import { DIRECTION, isTreeScrollable } from "../isScrollable";
 
 export interface PullToRefreshProps {
     pullDownContent: JSX.Element;
@@ -6,7 +7,7 @@ export interface PullToRefreshProps {
     refreshContent: JSX.Element;
     pullDownThreshold: number;
     onRefresh: () => Promise<any>;
-    triggerHeight?: number;
+    triggerHeight?: number | "auto";
     backgroundColor?: string;
 }
 
@@ -81,9 +82,29 @@ export class PullToRefresh extends React.Component<PullToRefreshProps, PullToRef
         const {triggerHeight = 40} = this.props;
         this.startY = e["pageY"] || e.touches[0].pageY;
         this.currentY = this.startY;
-        const top = this.container.getBoundingClientRect().top || this.container.getBoundingClientRect().y || 0;
-        if (this.startY - top > triggerHeight) {
-            return;
+
+        if (triggerHeight === "auto") {
+            const target = e.target;
+
+            const container = this.container;
+            if (!container) {
+                return;
+            }
+
+            // an element we're touching can be scrolled up, so gesture is going to be a scroll gesture
+            if (e.type === "touchstart" && isTreeScrollable(target, DIRECTION.up)) {
+                return;
+            }
+
+            // even though we're not scrolling, the pull-to-refresh isn't visible to the user so cancel
+            if (container.getBoundingClientRect().top < 0) {
+                return;
+            }
+        } else {
+            const top = this.container.getBoundingClientRect().top || this.container.getBoundingClientRect().y || 0;
+            if (this.startY - top > triggerHeight) {
+                return;
+            }
         }
 
         this.dragging = true;

src/isScrollable.ts

+
+export const DIRECTION = Object.freeze({
+    up: -0b01,
+    down: 0b01,
+});
+
+function isOverflowScrollable(element: Element) {
+    const overflowType = getComputedStyle(element).overflowY;
+    if (element === document.scrollingElement && overflowType === "visible") {
+        return true;
+    }
+
+    if (overflowType !== "scroll" && overflowType !== "auto") {
+        return false;
+    }
+
+    return true;
+}
+
+/**
+ * Returns whether a given element is scrollable in a given direction.
+ * This only checks this element, not any of its ancestors.
+ *
+ * @param {!Element} element The DOM element to check
+ * @param {!number} direction The direction (see {@link DIRECTION})
+ * @returns {!boolean} Whether the element is scrollable
+ */
+function isScrollable(element: Element, direction: number) {
+
+    if (!isOverflowScrollable(element)) {
+        return false;
+    }
+
+    if (direction === DIRECTION.down) {
+        const bottomScroll = element.scrollTop + element.clientHeight;
+
+        return bottomScroll < element.scrollHeight;
+    }
+
+    if (direction === DIRECTION.up) {
+        return element.scrollTop > 0;
+    }
+
+    throw new Error("unsupported direction");
+}
+
+/**
+ * Returns whether a given element or any of its ancestors (up to rootElement) is scrollable in a given direction.
+ *
+ * @param {!Element} element The leaf of the DOM tree to check
+ * @param {!number} dir The direction (see {@link DIRECTION})
+ * @returns {!boolean} Whether the element or one of its ancestors is scrollable.
+ */
+export function isTreeScrollable(element: Element, dir: number): boolean {
+    console.log(isScrollable(element, dir), element);
+    if (isScrollable(element, dir)) {
+        return true;
+    }
+
+    // if a body is overflow: hidden, scrolling will be disabled even though scrollingElement will report that it is not.
+    if (element === document.body && getComputedStyle(document.body).overflowY === "hidden") {
+        return false;
+    }
+
+    if (element.parentElement == null) {
+        return false;
+    }
+
+    return isTreeScrollable(element.parentElement, dir);
+}