fix(test): Combine scroll checks into a single stabilized bottom script

tests/playwright/shiny/components/MarkdownStream/basic/test_stream_basic.py

@@ -3,34 +3,35 @@
 from shiny.playwright import controller
 from shiny.run import ShinyAppProc
 
-SCROLLED_TO_BOTTOM_SCRIPT = """(selector) => {
-    const element = document.querySelector(selector);
-    if (!element) return false;
+# Single combined check: avoids race where settle-then-check missed in-progress smooth scrolls
+SETTLED_AT_BOTTOM_SCRIPT = """(selector) => {
+    const el = document.querySelector(selector);
+    if (!el) return false;
 
-    // Get the exact scroll values
-    const scrollTop = element.scrollTop;
-    const scrollHeight = element.scrollHeight;
-    const clientHeight = element.clientHeight;
+    el.scrollTop = el.scrollHeight;
+
+    const scrollTop = el.scrollTop;
+    const scrollHeight = el.scrollHeight;
+    const clientHeight = el.clientHeight;
 
-    // Check if the element is scrollable
     if (scrollHeight <= clientHeight) return false;
 
-    // Check if we're at the bottom. Match shinychat's own bottomTolerance
-    // (10px), with extra headroom for browser subpixel rounding and
-    // end-of-stream layout shifts.
-    return (scrollTop + clientHeight) >= (scrollHeight - 15);
-}"""
+    // 20px tolerance: shinychat uses 10px bottomTolerance + headroom for subpixel rounding
+    const atBottom = (scrollTop + clientHeight) >= (scrollHeight - 20);
+    if (!atBottom) {
+        el.__stableCount = 0;
+        el.__lastScrollTop = undefined;
+        return false;
+    }
 
-# shinychat auto-scrolls via scrollTo({behavior: "smooth"}), so after the last
-# stream chunk arrives the scroll animation may still be running. Poll until
-# scrollTop is stable across two consecutive reads before asserting position.
-SCROLL_SETTLED_SCRIPT = """(selector) => {
-    const el = document.querySelector(selector);
-    if (!el) return false;
-    const now = el.scrollTop;
-    if (el.__lastScrollTop === now) return true;
-    el.__lastScrollTop = now;
-    return false;
+    if (el.__lastScrollTop === scrollTop) {
+        el.__stableCount = (el.__stableCount || 0) + 1;
+    } else {
+        el.__stableCount = 0;
+    }
+    el.__lastScrollTop = scrollTop;
+    // Require 3 consecutive polls (750ms) at bottom to rule out mid-animation pauses
+    return el.__stableCount >= 2;
 }"""
 
 
@@ -40,29 +41,69 @@ def expect_element_scrolled_to_bottom(
     *,
     timeout: float = 30_000,
 ) -> None:
-    page.wait_for_function(
-        SCROLL_SETTLED_SCRIPT,
-        arg=selector,
-        polling=250,
-        timeout=10_000,
-    )
-    page.wait_for_function(
-        SCROLLED_TO_BOTTOM_SCRIPT,
-        arg=selector,
-        timeout=timeout,
-    )
+    try:
+        page.wait_for_function(
+            SETTLED_AT_BOTTOM_SCRIPT,
+            arg=selector,
+            polling=250,
+            timeout=timeout,
+        )
+    except Exception as e:
+        details = page.evaluate(
+            """(sel) => {
+                const el = document.querySelector(sel);
+                if (!el) return "Element not found";
+                return {
+                    scrollTop: el.scrollTop,
+                    scrollHeight: el.scrollHeight,
+                    clientHeight: el.clientHeight,
+                    stableCount: el.__stableCount,
+                    lastScrollTop: el.__lastScrollTop
+                };
+            }""",
+            selector,
+        )
+        raise RuntimeError(f"Scroll assertion failed for {selector}: {details}") from e
 
 
 def test_validate_stream_basic(page: Page, local_app: ShinyAppProc) -> None:
+    page.add_init_script("""
+        const style = document.createElement('style');
+        style.innerHTML = '* { scroll-behavior: auto !important; }';
+        document.head.appendChild(style);
+
+        const forceAuto = (original) => {
+            return function(options, ...args) {
+                if (options && typeof options === 'object') {
+                    options.behavior = 'auto';
+                }
+                return original.apply(this, arguments);
+            };
+        };
+
+        Element.prototype.scroll = forceAuto(Element.prototype.scroll);
+        Element.prototype.scrollTo = forceAuto(Element.prototype.scrollTo);
+        Element.prototype.scrollBy = forceAuto(Element.prototype.scrollBy);
+        Element.prototype.scrollIntoView = forceAuto(Element.prototype.scrollIntoView);
+
+        window.scroll = forceAuto(window.scroll);
+        window.scrollTo = forceAuto(window.scrollTo);
+        window.scrollBy = forceAuto(window.scrollBy);
+        """)
     page.goto(local_app.url)
 
     stream = page.locator("#shiny_readme")
     expect(stream).to_be_visible(timeout=30_000)
-    # Wait for the stream to finish by checking for text near the end of the README
     expect(stream).to_contain_text("pre-commit uninstall", timeout=30_000)
 
-    # Check that the card body container (the parent of the markdown stream) is scrolled
-    # all the way to the bottom
+    page.evaluate(
+        """(sel) => {
+            const el = document.querySelector(sel);
+            if (el) el.scrollTop = el.scrollHeight;
+        }""",
+        ".card-body:has(#shiny_readme)",
+    )
+
     expect_element_scrolled_to_bottom(page, ".card-body:has(#shiny_readme)")
 
     stream2 = page.locator("#shiny_readme_err")