[autocomplete] don't allow completions to overflow the box, or for the box to overflow the screen, see #999

ext/autocomplete/script.js

@@ -137,6 +137,7 @@ function renderCompletions(element) {
 		element.parentNode.insertBefore(completions_el, element.nextSibling);
 		let br = element.getBoundingClientRect();
 		completions_el.style.minWidth = br.width + 'px';
+		completions_el.style.maxWidth = 'calc(100vw - 2rem - ' + br.left + 'px)';
 		completions_el.style.left = window.scrollX + br.left + 'px';
 		completions_el.style.top = window.scrollY + (br.top + br.height) + 'px';
 	}

ext/autocomplete/style.css

@@ -10,6 +10,7 @@
     padding: 0;
     font-size: 1rem;
     white-space: nowrap;
+    overflow: hidden;
 }
 .autocomplete_completions LI {
     padding: 0.15em;