test(home): outline every fixed/sticky element to find the white bar

User reports: TEST #3 marker sits at the true bottom of the home
page's main content, but when scrolled the test text "goes behind"
an opaque white bar — confirming the bar is fixed/sticky (overlays
scrolling content). Our CSS only declares ONE fixed element near
the bottom (``#datatools-sticky-footer``), which the user already
ruled out. So something else — Streamlit native chrome, a third-
party widget, or a fixed element we haven't enumerated — is
overlaying the content.

Inject a small diagnostic iframe whose JS, running against the
parent document, walks every element on the page and outlines each
``position: fixed`` or ``position: sticky`` node with a distinct
color + a top-left label showing ``tagName#id[data-testid] pos=…
h=…px bg=…``. Re-runs after initial paint, on a couple of delays
(for late-mounting components), and on every scroll.

This is read-only — no DOM mutations beyond outline styles and
labels — so it's safe to ship even if I miss removing it.
The user can now visually identify which colored box is the
offending white bar and report its label.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/gui/_home.py

@@ -289,16 +289,87 @@ def _home_page() -> None:
                         key_namespace=name,
                     )
 
-    # TEMP: visible marker at the very end of the home page's main
-    # content. Used to locate the "white bar" the user wants removed.
-    # If this red banner lines up with the offending white strip when
-    # scrolled to the bottom, the strip is something rendered right
-    # here at the tail of ``stAppViewBlockContainer``.
+    # TEMP: end-of-content marker — confirmed at the true bottom of
+    # the home page's main content. The user reports content scrolls
+    # *behind* the offending white bar, so the bar is fixed-positioned.
+    # Only ``#datatools-sticky-footer`` is fixed at the bottom per
+    # our CSS, but the user already confirmed the sticky footer is
+    # NOT the offending bar. So there's a fixed element we haven't
+    # accounted for. The JS below outlines EVERY fixed/sticky element
+    # in the parent document with a labelled colored border so we can
+    # see exactly what's overlaying scrolled content.
     st.markdown(
         '<div style="background:#ffe0e0;border:2px dashed #b00020;'
         'color:#b00020;font-weight:700;font-size:13px;text-align:center;'
         'padding:8px 12px;margin:4px 0;font-family:system-ui,sans-serif;">'
-        '◀ CLAUDE TEST #3 — END OF MAIN CONTENT — IS THIS THE WHITE BAR? ▶'
+        '◀ CLAUDE TEST #3 — END OF MAIN CONTENT ▶'
         '</div>',
         unsafe_allow_html=True,
     )
+    st.iframe(
+        """
+<script>
+(function () {
+  function outlineFixed() {
+    var doc = window.parent.document;
+    if (!doc) return;
+    // Clear previous outlines + labels so reruns don't stack.
+    doc.querySelectorAll('[data-claude-outline-label]').forEach(function (n) {
+      n.remove();
+    });
+    doc.querySelectorAll('[data-claude-outlined]').forEach(function (el) {
+      el.style.outline = '';
+      el.style.outlineOffset = '';
+      el.removeAttribute('data-claude-outlined');
+    });
+    var palette = ['#e6194b', '#3cb44b', '#4363d8', '#f58231', '#911eb4',
+                   '#46f0f0', '#f032e6', '#bcf60c', '#fabebe', '#008080'];
+    var idx = 0;
+    doc.querySelectorAll('*').forEach(function (el) {
+      var cs = window.parent.getComputedStyle(el);
+      if (cs.position !== 'fixed' && cs.position !== 'sticky') return;
+      var rect = el.getBoundingClientRect();
+      // Skip zero-size and off-screen-only items.
+      if (rect.width < 2 || rect.height < 2) return;
+      var color = palette[idx % palette.length];
+      idx += 1;
+      el.style.outline = '3px solid ' + color;
+      el.style.outlineOffset = '-3px';
+      el.setAttribute('data-claude-outlined', '1');
+      // Label pinned to the element's top-left.
+      var label = doc.createElement('div');
+      label.setAttribute('data-claude-outline-label', '1');
+      var name = el.tagName.toLowerCase()
+        + (el.id ? '#' + el.id : '')
+        + (el.getAttribute('data-testid')
+              ? '[' + el.getAttribute('data-testid') + ']' : '')
+        + ' pos=' + cs.position
+        + ' h=' + Math.round(rect.height) + 'px'
+        + ' bg=' + cs.backgroundColor;
+      label.textContent = name;
+      label.style.cssText =
+        'position:fixed;left:' + Math.max(0, rect.left) + 'px;'
+        + 'top:' + Math.max(0, rect.top - 18) + 'px;'
+        + 'background:' + color + ';color:#fff;'
+        + 'font:11px/1.2 ui-monospace,monospace;'
+        + 'padding:2px 5px;border-radius:3px;'
+        + 'z-index:2147483647;pointer-events:none;'
+        + 'white-space:nowrap;max-width:90vw;overflow:hidden;'
+        + 'text-overflow:ellipsis;';
+      doc.body.appendChild(label);
+    });
+  }
+  // Run after initial paint and again after Streamlit's component
+  // iframes settle.
+  setTimeout(outlineFixed, 400);
+  setTimeout(outlineFixed, 1500);
+  setTimeout(outlineFixed, 3500);
+  // Re-run on scroll so labels track the current viewport state.
+  window.parent.addEventListener('scroll', function () {
+    setTimeout(outlineFixed, 50);
+  }, true);
+})();
+</script>
+""",
+        height=1,
+    )