diff --git a/documentation/source/_themes/m21/static/copybutton.js b/documentation/source/_themes/m21/static/copybutton.js
index c9f49770b1..71d8e99147 100644
--- a/documentation/source/_themes/m21/static/copybutton.js
+++ b/documentation/source/_themes/m21/static/copybutton.js
@@ -1,64 +1,174 @@
 // found in the _themes/m21/static folder
+// MSAC: I can't remember where this came from, but in 2026 rewritten
+// to use modern JS and no jQuery
 
-$(document).ready(function() {
-	/* Add a [>>>] button on the top-right corner of code samples to hide
-	 * the >>> and ... prompts and the output and thus make the code
-	 * copyable. */
-	var div = $('.highlight-python .highlight,' +
-		    '.highlight-python3 .highlight,' +
-		    '.highlight-default .highlight');
-	var pre = div.find('pre');
+/** Add a [>>>] button on the top-right corner of code samples to hide
+ * the >>> and ... prompts and the output and thus make the code
+ * copyable. */
+document.addEventListener('DOMContentLoaded', () => {
+    const divs = document.querySelectorAll(
+        '.highlight-python .highlight,'
+        + '.highlight-python3 .highlight,'
+        + '.highlight-default .highlight'
+    );
 
-	// get the styles from the current theme
-	pre.parent().parent().css('position', 'relative');
-	var hide_text = 'Hide the prompts and output';
-	var show_text = 'Show the prompts and output';
-	var border_width = pre.css('border-top-width');
-	var border_style = pre.css('border-top-style');
-	var border_color = pre.css('border-top-color');
-	var button_styles = {
-	    'cursor':'pointer', 'position': 'absolute', 'top': '0', 'right': '0',
-	    'border-color': border_color, 'border-style': border_style,
-	    'border-width': border_width, 'color': border_color, 'text-size': '75%',
-	    'font-family': 'monospace', 'padding-left': '0.2em', 'padding-right': '0.2em',
-	    'border-radius': '0 3px 0 0'
-	}
+    // We take the first <pre> we find (if any) to read theme styles
+    // and apply them to the buttons
+    let firstPre = null;
+    for (const this_div of divs) {
+        const maybePre = this_div.querySelector('pre');
+        if (maybePre) {
+            firstPre = maybePre;
+            break;
+        }
+    }
 
-	// create and add the button to all the code blocks that contain >>>
-	div.each(function(index) {
-		var jthis = $(this);
-		if (jthis.find('.gp').length > 0) {
-		    var button = $('<span class="copybutton">&gt;&gt;&gt;</span>');
-		    button.css(button_styles)
-			button.attr('title', hide_text);
-		    button.data('hidden', 'false');
-		    jthis.prepend(button);
-		}
-		// tracebacks (.gt) contain bare text elements that need to be
-		// wrapped in a span to work with .nextUntil() (see later)
-		jthis.find('pre:has(.gt)').contents().filter(function() {
-			return ((this.nodeType == 3) && (this.data.trim().length > 0));
-		    }).wrap('<span>');
-	    });
+    // get the styles from the current theme
+    if (firstPre && firstPre.parentElement && firstPre.parentElement.parentElement) {
+        firstPre.parentElement.parentElement.style.position = 'relative';
+    }
+    const hide_text = 'Hide the prompts and output';
+    const show_text = 'Show the prompts and output';
 
-	// define the behavior of the button when it's clicked
-	$('.copybutton').click(function(e){
-		e.preventDefault();
-		var button = $(this);
-		if (button.data('hidden') === 'false') {
-		    // hide the code output
-		    button.parent().find('.go, .gp, .gt').hide();
-		    button.next('pre').find('.gt').nextUntil('.gp, .go').css('visibility', 'hidden');
-		    button.css('text-decoration', 'line-through');
-		    button.attr('title', show_text);
-		    button.data('hidden', 'true');
-		} else {
-		    // show the code output
-		    button.parent().find('.go, .gp, .gt').show();
-		    button.next('pre').find('.gt').nextUntil('.gp, .go').css('visibility', 'visible');
-		    button.css('text-decoration', 'none');
-		    button.attr('title', hide_text);
-		    button.data('hidden', 'false');
-		}
-	    });
-    });
+    let border_width = '';
+    let border_style = '';
+    let border_color = '';
+    if (firstPre) {
+        const cs = window.getComputedStyle(firstPre);
+        border_width = cs.borderTopWidth;
+        border_style = cs.borderTopStyle;
+        border_color = cs.borderTopColor;
+    }
+
+    function apply_button_styles(button) {
+        button.style.cursor = 'pointer';
+        button.style.position = 'absolute';
+        button.style.top = '0';
+        button.style.right = '0';
+        button.style.borderColor = border_color;
+        button.style.borderStyle = border_style;
+        button.style.borderWidth = border_width;
+        button.style.color = border_color;
+        button.style.fontSize = '75%';
+        button.style.fontFamily = 'monospace';
+        button.style.paddingLeft = '0.2em';
+        button.style.paddingRight = '0.2em';
+        button.style.borderRadius = '0 3px 0 0';
+    }
+
+    function hide_elements(parent, selector) {
+        const els = parent.querySelectorAll(selector);
+        for (const el of els) {
+            el.style.display = 'none';
+        }
+    }
+
+    function show_elements(parent, selector) {
+        const els = parent.querySelectorAll(selector);
+        for (const el of els) {
+            el.style.display = '';
+        }
+    }
+
+    function set_traceback_visibility(pre, visible) {
+        // Equivalent to: button.next('pre').find('.gt').nextUntil('.gp, .go')...
+        const gts = pre.querySelectorAll('.gt');
+        for (const gt of gts) {
+            let n = gt.nextSibling;
+            while (n) {
+                if (n.nodeType === Node.ELEMENT_NODE) {
+                    const el = n;
+                    if (el.classList.contains('gp') || el.classList.contains('go')) {
+                        break;
+                    }
+                    el.style.visibility = visible ? 'visible' : 'hidden';
+                }
+                n = n.nextSibling;
+            }
+        }
+    }
+
+    /**
+     * find the next sibling that is a <pre> tag.
+     */
+    function next_pre_sibling(startEl) {
+        let next = startEl.nextElementSibling;
+        while (next && next.tagName.toLowerCase() !== 'pre') {
+            next = next.nextElementSibling;
+        }
+        return next;
+    }
+
+    // create and add the button to all the code blocks that contain >>>
+    for (const this_div of divs) {
+        // get the styles from the current theme (per-block positioning like before)
+        const pre = this_div.querySelector('pre');
+        if (pre && pre.parentElement && pre.parentElement.parentElement) {
+            pre.parentElement.parentElement.style.position = 'relative';
+        }
+
+        if (this_div.querySelectorAll('.gp').length > 0) {
+            const button = document.createElement('span');
+            button.className = 'copy_button';
+            button.textContent = '>>>';
+            button.setAttribute('role', 'button');
+            button.setAttribute('tabindex', '0');
+            apply_button_styles(button);
+            button.setAttribute('title', hide_text);
+            button.setAttribute('aria-pressed', 'false');
+            this_div.insertBefore(button, this_div.firstChild);
+        }
+
+        // tracebacks (.gt) contain bare text elements that need to be
+        // wrapped in a span to work with .nextUntil() (see later)
+        const preWithGt = this_div.querySelectorAll('pre');
+        for (const preNode of preWithGt) {
+            if (preNode.querySelector('.gt')) {
+                const contents = Array.from(preNode.childNodes);
+                for (const node of contents) {
+                    if ((node.nodeType === Node.TEXT_NODE) && node.data.trim()) {
+                        const span = document.createElement('span');
+                        span.textContent = node.data;
+                        preNode.replaceChild(span, node);
+                    }
+                }
+            }
+        }
+    }
+
+    // define the behavior of the button when it's clicked
+    const buttons = document.querySelectorAll('.copy_button');
+    for (const button of buttons) {
+        button.addEventListener('click', e => {
+            e.preventDefault();
+            const parent = button.parentNode;
+            const pre = next_pre_sibling(button);
+            if (button.getAttribute('aria-pressed') === 'false') {
+                // hide the code output
+                hide_elements(parent, '.go, .gp, .gt');
+                if (pre) {
+                    set_traceback_visibility(pre, false);
+                }
+                button.style.textDecoration = 'line-through';
+                button.setAttribute('title', show_text);
+                button.setAttribute('aria-pressed', 'true');
+            } else {
+                // show the code output
+                show_elements(parent, '.go, .gp, .gt');
+                if (pre) {
+                    // pre is the same thing jQuery would return for .next('pre')
+                    set_traceback_visibility(pre, true);
+                }
+                button.style.textDecoration = 'none';
+                button.setAttribute('title', hide_text);
+                button.setAttribute('aria-pressed', 'false');
+            }
+        });
+        button.addEventListener('keydown', e => {
+            if (e.key === 'Enter' || e.key === ' ') {
+                e.preventDefault();
+                button.click();
+            }
+        });
+    }
+});