Only make scrollable code blocks into tab stops (#1777)

* Only make scrollable code blocks into tab stops

* Check for y-overflow too

src/pydata_sphinx_theme/assets/scripts/pydata-sphinx-theme.js

@@ -692,6 +692,32 @@ function setupMobileSidebarKeyboardHandlers() {
   });
 }
 
+/**
+ * When the page loads or the window resizes check all elements with
+ * [data-tabindex="0"], and if they have scrollable overflow, set tabIndex = 0.
+ */
+function setupLiteralBlockTabStops() {
+  const updateTabStops = () => {
+    document.querySelectorAll('[data-tabindex="0"]').forEach((el) => {
+      el.tabIndex =
+        el.scrollWidth > el.clientWidth || el.scrollHeight > el.clientHeight
+          ? 0
+          : -1;
+    });
+  };
+  window.addEventListener("resize", debounce(updateTabStops, 300));
+  updateTabStops();
+}
+function debounce(callback, wait) {
+  let timeoutId = null;
+  return (...args) => {
+    clearTimeout(timeoutId);
+    timeoutId = setTimeout(() => {
+      callback(...args);
+    }, wait);
+  };
+}
+
 /*******************************************************************************
  * Call functions after document loading.
  */
@@ -703,3 +729,4 @@ documentReady(setupSearchButtons);
 documentReady(initRTDObserver);
 documentReady(setupMobileSidebarKeyboardHandlers);
 documentReady(fixMoreLinksInMobileSidebar);
+documentReady(setupLiteralBlockTabStops);

src/pydata_sphinx_theme/translator.py

@@ -33,7 +33,7 @@ def starttag(self, *args, **kwargs):
             kwargs["ARIA-LEVEL"] = "2"
 
         if "pre" in args:
-            kwargs["tabindex"] = "0"
+            kwargs["data-tabindex"] = "0"
 
         return super().starttag(*args, **kwargs)
 
@@ -50,7 +50,7 @@ def visit_literal_block(self, node):
             # executed successfully and appended to self.body a string of HTML
             # representing the code block, which we then modify.
             html_string = self.body[-1]
-            self.body[-1] = html_string.replace("<pre", '<pre tabindex="0"')
+            self.body[-1] = html_string.replace("<pre", '<pre data-tabindex="0"')
             raise nodes.SkipNode
 
     def visit_table(self, node):

tests/test_a11y.py

@@ -240,3 +240,25 @@ def test_version_switcher_highlighting(page: Page, url_base: str) -> None:
         light_mode = "rgb(10, 125, 145)"  # pst-color-primary
         # dark_mode = "rgb(63, 177, 197)"
         expect(entry).to_have_css("color", light_mode)
+
+
+def test_code_block_tab_stop(page: Page, url_base: str) -> None:
+    """Code blocks that have scrollable content should be tab stops."""
+    page.set_viewport_size({"width": 1440, "height": 720})
+    page.goto(urljoin(url_base, "/examples/kitchen-sink/blocks.html"))
+    code_block = page.locator(
+        'css=#code-block pre[data-tabindex="0"]', has_text="from typing import Iterator"
+    )
+
+    # Viewport is wide, so code block content fits, no overflow, no tab stop
+    assert code_block.evaluate("el => el.scrollWidth > el.clientWidth") is False
+    assert code_block.evaluate("el => el.tabIndex") != 0
+
+    page.set_viewport_size({"width": 400, "height": 720})
+
+    # Resize handler is debounced with 300 ms wait time
+    page.wait_for_timeout(301)
+
+    # Narrow viewport, content overflows and code block should be a tab stop
+    assert code_block.evaluate("el => el.scrollWidth > el.clientWidth") is True
+    assert code_block.evaluate("el => el.tabIndex") == 0