Fix: Handle start_time changes in st.video (streamlit#7257)

Adds additional check to useEffect to handle changes in start_time for st.video

e2e/scripts/st_video.py

@@ -19,3 +19,7 @@
 url = "https://www.w3schools.com/html/mov_bbb.mp4"
 file = requests.get(url).content
 st.video(file)
+
+# Test start time with widget
+timestamp = st.number_input("Start Time (in seconds)", min_value=0, value=6)
+st.video(url, start_time=int(timestamp))

e2e/specs/st_video.spec.js

@@ -22,6 +22,21 @@ describe("st.video", () => {
   });
 
   it("displays a video player", () => {
-    cy.get(".element-container .stVideo").should("have.attr", "src");
+    cy.getIndexed(".element-container .stVideo", 0).should("have.attr", "src");
+  });
+
+  it("handles a start time", () => {
+    cy.getIndexed(".element-container .stVideo", 1).should("have.attr", "src");
+  });
+
+  it("handles changes in start time", () => {
+    // Change the start time from 6 to 5
+    cy.get(".element-container .stNumberInput .step-down").click();
+
+    // Wait for the video start time to update
+    cy.wait(3000);
+
+    // Confirm video updated
+    cy.getIndexed(".element-container .stVideo", 1).matchImageSnapshot("video-updated-start");
   });
 });

frontend/lib/src/components/elements/Video/Video.tsx

@@ -35,7 +35,14 @@ export default function Video({
 
   /* Element may contain "url" or "data" property. */
 
-  const { type, url } = element
+  const { type, url, startTime } = element
+
+  // Handle startTime changes
+  useEffect(() => {
+    if (videoRef.current) {
+      videoRef.current.currentTime = startTime
+    }
+  }, [startTime])
 
   useEffect(() => {
     const videoNode = videoRef.current