fix: offset scrollspy so righthand-nav highlights the correct entry (#3820)

Also rewrites scroll_to_element() using javascript. This seems to be less flaky
than the ActionChains implementation. Slight change in behavior - now scrolls
the requested element to the middle of the window instead of barely into view.

ietf/meeting/tests_js.py

@@ -1529,10 +1529,8 @@ class AgendaTests(IetfSeleniumTestCase):
             ),
             'Modal open button not found or not clickable',
         )
-        # FIXME: no idea why we need js instead of the following:
-        # self.scroll_to_element(open_modal_button)
-        # open_modal_button.click()
-        self.driver.execute_script("arguments[0].click();", open_modal_button)
+        self.scroll_to_element(open_modal_button)
+        open_modal_button.click()
         WebDriverWait(self.driver, 2).until(
             expected_conditions.visibility_of(modal_div),
             'Modal did not become visible after clicking open button',
@@ -1586,6 +1584,7 @@ class AgendaTests(IetfSeleniumTestCase):
         )
         self.scroll_to_element(open_modal_button)
         open_modal_button.click()
+        # self.driver.execute_script("arguments[0].click();", open_modal_button)
         WebDriverWait(self.driver, 2).until(
             expected_conditions.visibility_of(modal_div),
             'Modal did not become visible after clicking open button for refresh test',

ietf/static/js/ietf.js

@@ -251,9 +251,13 @@ $(function () {
                     }
                 });
 
+            // offset the scrollspy to account for the menu bar
+            const contentOffset = contentElement ? contentElement.offset().top : 0;
+
             $("body")
                 .attr("data-bs-spy", "scroll")
                 .attr("data-bs-target", "#righthand-nav")
+                .attr("data-bs-offset", contentOffset)
                 .scrollspy("refresh");
 
         }

ietf/utils/jstest.py

@@ -11,7 +11,6 @@ try:
     from selenium import webdriver
     from selenium.webdriver.chrome.service import Service
     from selenium.webdriver.chrome.options import Options
-    from selenium.webdriver.common.action_chains import ActionChains
     from selenium.webdriver.common.by import By
     from selenium.webdriver.common.desired_capabilities import DesiredCapabilities
 except ImportError as e:
@@ -86,8 +85,16 @@ class IetfSeleniumTestCase(IetfLiveServerTestCase):
 
     def scroll_to_element(self, element):
         """Scroll an element into view"""
-        actions = ActionChains(self.driver)
-        actions.move_to_element(element).perform()
+        # Compute the offset to put the element in the center of the window
+        win_height = self.driver.get_window_rect()['height']
+        offset = element.rect['y'] + (element.rect['height'] - win_height) // 2
+        self.driver.execute_script(
+            'window.scroll({top: arguments[0], behavior: "instant"})',
+            offset,
+        )
+        # The ActionChains approach below seems to be fragile, hence he JS above.
+        # actions = ActionChains(self.driver)
+        # actions.move_to_element(element).perform()
 
 
 class presence_of_element_child_by_css_selector: