fix: keyboard-visible delete, valid list semantics, undo support

The delete button was opacity:0 and only revealed on hover, so tabbing to
it showed nothing at all. The focus outline was being drawn on an
invisible element. It shows on :focus-within now too.

The row was role="button" with tabindex=0 while containing two real
buttons, which isn't valid. Rows are <li> in a <ul> now, so the list gets
announced with its count and position, and the checkbox carries the task
text plus aria-pressed.

Backspace no longer deletes. It's the key people hit meaning "go back",
and there was no way to get the task back afterwards. Delete still works,
and Cmd/Ctrl+Z reverses it.

Enter and Escape move from window to the input. On window, pressing Enter
with a task focused toggled that task and also added whatever was sitting
in the input.

src/App.svelte

@@ -8,8 +8,10 @@
   import { createStorage } from './lib/storage.js';
   import { rollover, msUntilNextMidnight } from './lib/rollover.js';
   import * as taskOps from './lib/tasks.js';
+  import { createUndoStack, applyUndo } from './lib/undo.js';
 
   const storage = createStorage();
+  const undoStack = createUndoStack();
 
   let tasks = $state([]);
   let newTask = $state('');
@@ -43,27 +45,55 @@
   }
 
   function deleteTask(id) {
+    const index = tasks.findIndex(task => task.id === id);
+    if (index === -1) return;
+    undoStack.push({ type: 'delete', task: tasks[index], index });
     setTasks(taskOps.deleteTask(tasks, id));
   }
 
   function clearCompleted() {
+    // Built in ascending index order, which applyUndo relies on to put each
+    // task back where it was.
+    const removed = tasks
+      .map((task, index) => ({ task, index }))
+      .filter(({ task }) => task.completed);
+
+    if (removed.length === 0) return;
+    undoStack.push({ type: 'clearCompleted', removed });
     setTasks(taskOps.clearCompleted(tasks));
   }
 
+  function undo() {
+    const entry = undoStack.pop();
+    if (entry) setTasks(applyUndo(tasks, entry));
+  }
+
   function toggleTheme() {
     darkMode = !darkMode;
   }
 
-  function handleKeydown(event) {
+  /** Scoped to the input. Previously this also sat on window, so Enter while a
+   *  task was focused would toggle that task *and* add whatever was in the
+   *  input. */
+  function handleInputKeydown(event) {
     if (event.key === 'Enter') addTask();
     else if (event.key === 'Escape') newTask = '';
   }
 
+  function handleGlobalKeydown(event) {
+    if (!(event.metaKey || event.ctrlKey) || event.key.toLowerCase() !== 'z') return;
+    // Leave the text field its own native undo.
+    if (event.target instanceof HTMLInputElement) return;
+    event.preventDefault();
+    undo();
+  }
+
+  /** Backspace no longer deletes: it is the key people press meaning "go back",
+   *  and a task destroyed that way used to be unrecoverable. Delete still does,
+   *  and Cmd/Ctrl+Z now reverses it. */
   function handleTaskKeydown(event, taskId) {
-    if (event.key === ' ' || event.key === 'Enter') {
+    if (event.key === 'Delete') {
       event.preventDefault();
-      toggleTask(taskId);
-    } else if (event.key === 'Delete' || event.key === 'Backspace') {
       deleteTask(taskId);
     }
   }
@@ -170,7 +200,7 @@
   });
 </script>
 
-<svelte:window onkeydown={handleKeydown} />
+<svelte:window onkeydown={handleGlobalKeydown} />
 
 <div class="app">
   <header class="header">
@@ -235,7 +265,7 @@
           type="text"
           placeholder="+ Add a task"
           class="task-input"
-          onkeydown={handleKeydown}
+          onkeydown={handleInputKeydown}
         />
       </div>
 
@@ -271,8 +301,14 @@
               <p>No tasks yet. Add one above to get started!</p>
             </div>
           {:else}
+            <ul class="task-items">
             {#each tasks as task, index (task.id)}
-              <div 
+              <!-- The row is not itself focusable. Its handlers act on events
+                   bubbling up from the buttons inside it, and every action they
+                   provide is reachable from the keyboard: Delete removes a task,
+                   Alt+Arrow reorders one. -->
+              <!-- svelte-ignore a11y_no_noninteractive_element_interactions -->
+              <li
                 class="task-item"
                 class:completed={task.completed}
                 class:dragging={draggedItem === index}
@@ -285,15 +321,13 @@
                 ondragend={handleDragEnd}
                 ondragleave={handleDragLeave}
                 onkeydown={(e) => handleTaskKeydown(e, task.id)}
-                tabindex="0"
-                role="button"
-                aria-label={task.completed ? `Completed: ${task.text}` : `Incomplete: ${task.text}`}
               >
                 <button
                   class="checkbox"
                   class:checked={task.completed}
                   onclick={() => toggleTask(task.id)}
-                  aria-label={task.completed ? 'Mark as incomplete' : 'Mark as complete'}
+                  aria-pressed={task.completed}
+                  aria-label={task.text}
                 >
                   {#if task.completed}
                     <svg class="checkmark" viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
@@ -307,7 +341,7 @@
                 <button
                   class="delete-btn"
                   onclick={() => deleteTask(task.id)}
-                  aria-label="Delete task"
+                  aria-label="Delete {task.text}"
                 >
                   <svg viewBox="0 0 24 24" fill="none" xmlns="http://www.w3.org/2000/svg">
                     <polyline points="3,6 5,6 21,6" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
@@ -316,8 +350,9 @@
                     <line x1="14" y1="11" x2="14" y2="17" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/>
                   </svg>
                 </button>
-              </div>
+              </li>
             {/each}
+            </ul>
           {/if}
         {/key}
       </div>

src/style.css

@@ -242,11 +242,18 @@ body {
 }
 
 .task-list {
+  min-height: 400px;
+  position: relative;
+}
+
+/* A real list, so screen readers announce item counts and position. */
+.task-items {
   display: flex;
   flex-direction: column;
   gap: 12px;
-  min-height: 400px;
-  position: relative;
+  list-style: none;
+  margin: 0;
+  padding: 0;
 }
 
 .task-item {
@@ -362,7 +369,11 @@ body {
   flex-shrink: 0;
 }
 
-.task-item:hover .delete-btn {
+/* focus-within matters as much as hover here: the button is opacity 0 by
+   default, so a keyboard user tabbing to it previously saw nothing at all —
+   the focus outline was drawn on an invisible element. */
+.task-item:hover .delete-btn,
+.task-item:focus-within .delete-btn {
   opacity: 1;
 }