Fix empty anchor scrolling (streamlit#9206)

## Describe your changes

The page currently auto-scrolls if there is an empty header element,
even if no anchor was used via window hash. This PR fixes the issue by
only applying the anchor scroll if there is actually a window hash.

## GitHub Issue Link (if applicable)

- Closes streamlit#9203

## Testing Plan

- Added e2e test.

---

**Contribution License Agreement**

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project are made under the Apache 2.0 license.

e2e_playwright/st_heading.py

@@ -56,3 +56,6 @@
 for color in colors:
     st.subheader(f"{color.capitalize()} Subheader Divider:", divider=color)
     st.write(lorem_ipsum_text)
+
+# Empty subheader to test correct anchor behavior:
+st.subheader("")

e2e_playwright/st_heading_test.py

@@ -70,7 +70,7 @@ def test_correct_number_and_content_of_subheader_elements(app: Page):
     subheaders = _get_subheader_elements(app).filter(
         has_not_text=_subheader_divider_filter_text
     )
-    expect(subheaders).to_have_count(7)
+    expect(subheaders).to_have_count(8)
 
     expect(subheaders.nth(0)).to_have_text("info This subheader is awesome!")
     expect(subheaders.nth(1)).to_have_text("This subheader is awesome too!")
@@ -269,3 +269,16 @@ def test_help_tooltip_works(app: Page):
 
     title_with_help = _get_title_elements(app).nth(1)
     expect_help_tooltip(app, title_with_help, tooltip_text)
+
+
+def test_not_scrolled_on_empty_anchor_tag(app: Page):
+    """Test that the page is not scrolled when the page contains an empty
+    header/anchor tag and no window hash."""
+
+    # Check if the page is still scrolled to the top
+    # after one second timeout.
+    app.wait_for_timeout(1000)
+    scroll_position = app.evaluate("window.scrollY")
+    # Usage of assert is fine here since we just need to verify that
+    # this is still scrolled to top, no need to wait for this to happen.
+    assert scroll_position == 0

frontend/lib/src/components/shared/StreamlitMarkdown/StreamlitMarkdown.tsx

@@ -214,7 +214,8 @@ export const HeadingWithActionElements: FunctionComponent<
 
       const anchor = propsAnchor || createAnchorFromText(node.textContent)
       setElementId(anchor)
-      if (window.location.hash.slice(1) === anchor) {
+      const windowHash = window.location.hash.slice(1)
+      if (windowHash && windowHash === anchor) {
         setTarget(node)
       }
     },