feat(flowchart): keyboard shortcuts overlay (C19)

After 17 commits of feature work the standalone window had a lot of
hidden keyboard shortcuts (Cmd+Z, Cmd+C/V/D, Delete, Alt+drag, etc.)
that no user could discover. This adds:

  - 'Shortcuts (?)' toolbar button next to Export Visio.
  - '?' key (or Shift+/) toggles the overlay; Esc closes it.
  - A read-only modal that lists every canvas-relevant shortcut in
    a 2-column table (Action / Shortcut) with <kbd> tags for the
    key combos, styled to look like physical keys.
  - Click on the dimmed overlay backdrop also closes it.
  - Reuses the existing .fc-modal fade + scale-in animation.

The overlay sits inside #fc-left so it overlays the canvas, not the
side panel. Hidden by default via the HTML hidden attribute.

Implementation: btnHelp / shortcutsModal / shortcutsOverlay /
shortcutsClose are added to the bundle's els bag. A small
showShortcutsOverlay / hideShortcutsOverlay pair toggles their
.hidden. The keydown handler gets two new branches at the top:

  - Esc: close the overlay if open.
  - ? (computed from ev.key === '?' or 'Shift+/', no modifier
    keys held): toggle the overlay.

Both branches return early so the existing Cmd+Z / Cmd+C / etc.
handlers below them still work.

Tests: 3 new (1386 total) covering the toolbar toggle, the
? + Esc keyboard flow, and that every shortcut the user can
trigger is listed in the modal.

Amit Haridas

src/flowchart-generator.html

@@ -256,6 +256,50 @@
           opacity: 1;
         }
       }
+      /* v4.13.0 — keyboard shortcuts modal + kbd tags. The modal
+         reuses the existing .fc-modal animation; the table lays
+         out Action / Shortcut columns, with <kbd> tags for the
+         key combos styled to look like physical keys. */
+      .fc-shortcuts-modal {
+        max-width: 460px;
+        width: 90vw;
+      }
+      .fc-shortcuts-table {
+        width: 100%;
+        border-collapse: collapse;
+        margin: 12px 0 16px;
+        font-size: 0.9rem;
+      }
+      .fc-shortcuts-table th {
+        text-align: left;
+        font-weight: 600;
+        padding: 4px 0;
+        border-bottom: 1px solid var(--fc-border, #d1d5db);
+        color: #4b5563;
+        font-size: 0.75rem;
+        text-transform: uppercase;
+        letter-spacing: 0.05em;
+      }
+      .fc-shortcuts-table td {
+        padding: 6px 8px;
+        border-bottom: 1px solid #f3f4f6;
+      }
+      .fc-shortcuts-table td:last-child {
+        text-align: right;
+        white-space: nowrap;
+      }
+      kbd {
+        display: inline-block;
+        padding: 1px 6px;
+        font-family: ui-monospace, SFMono-Regular, 'SF Mono', Menlo, monospace;
+        font-size: 0.78rem;
+        background: #f3f4f6;
+        border: 1px solid #d1d5db;
+        border-bottom-width: 2px;
+        border-radius: 3px;
+        color: #1f2937;
+        margin: 0 1px;
+      }
       /* v4.10.0 — preview-render info card explaining the layout */
       .fc-preview-render-note {
         font-size: 0.75rem;
@@ -394,6 +438,80 @@
     <div class="fc-split">
       <div class="fc-left">
         <div class="fc-canvas-host" id="canvas-host"></div>
+        <!-- v4.13.0 — keyboard shortcuts overlay. Triggered by the
+             "Shortcuts (?)" toolbar button or the ? key. Hidden by
+             default; .open toggles visibility + the fade-in animation. -->
+        <div class="fc-modal-overlay" id="fc-shortcuts-overlay" hidden></div>
+        <div
+          class="fc-modal fc-shortcuts-modal"
+          id="fc-shortcuts-modal"
+          role="dialog"
+          aria-labelledby="fc-shortcuts-title"
+          hidden
+        >
+          <h2 id="fc-shortcuts-title">Keyboard Shortcuts</h2>
+          <table class="fc-shortcuts-table">
+            <thead>
+              <tr>
+                <th>Action</th>
+                <th>Shortcut</th>
+              </tr>
+            </thead>
+            <tbody>
+              <tr>
+                <td>Undo</td>
+                <td><kbd>Ctrl/⌘</kbd>+<kbd>Z</kbd></td>
+              </tr>
+              <tr>
+                <td>Redo</td>
+                <td><kbd>Ctrl/⌘</kbd>+<kbd>Shift</kbd>+<kbd>Z</kbd></td>
+              </tr>
+              <tr>
+                <td>Copy selection</td>
+                <td><kbd>Ctrl/⌘</kbd>+<kbd>C</kbd></td>
+              </tr>
+              <tr>
+                <td>Paste</td>
+                <td><kbd>Ctrl/⌘</kbd>+<kbd>V</kbd></td>
+              </tr>
+              <tr>
+                <td>Duplicate</td>
+                <td><kbd>Ctrl/⌘</kbd>+<kbd>D</kbd></td>
+              </tr>
+              <tr>
+                <td>Delete selected</td>
+                <td><kbd>Delete</kbd> / <kbd>Backspace</kbd></td>
+              </tr>
+              <tr>
+                <td>Connect from a node</td>
+                <td><kbd>Alt</kbd>+drag</td>
+              </tr>
+              <tr>
+                <td>Pan view</td>
+                <td><kbd>Shift</kbd>+drag on background</td>
+              </tr>
+              <tr>
+                <td>Multi-select</td>
+                <td><kbd>Shift</kbd>+click / drag-rect</td>
+              </tr>
+              <tr>
+                <td>Zoom in / out</td>
+                <td><kbd>Ctrl</kbd>+wheel</td>
+              </tr>
+              <tr>
+                <td>Reset All</td>
+                <td>(toolbar button)</td>
+              </tr>
+              <tr>
+                <td>Show this overlay</td>
+                <td><kbd>?</kbd></td>
+              </tr>
+            </tbody>
+          </table>
+          <div class="fc-modal-actions">
+            <button class="fc-btn primary" id="fc-shortcuts-close">Close (Esc)</button>
+          </div>
+        </div>
         <!--
           v4.11.0 — button-driven UI. Every mutation (add/delete node,
           add/delete edge, change label, change kind) goes through the
@@ -552,6 +670,13 @@
             >
               Export Visio
             </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-help"
+              title="Show keyboard shortcuts (press ? any time)"
+            >
+              Shortcuts (?)
+            </button>
           </div>
           <span id="fc-status" aria-live="polite"></span>
         </div>

src/renderer/flowchart-bundle.js

@@ -1212,6 +1212,10 @@
     btnDistributeH: document.getElementById('fc-btn-distribute-h'),
     btnDistributeV: document.getElementById('fc-btn-distribute-v'),
     btnSelectAll: document.getElementById('fc-btn-select-all'),
+    btnHelp: document.getElementById('fc-btn-help'),
+    shortcutsModal: document.getElementById('fc-shortcuts-modal'),
+    shortcutsOverlay: document.getElementById('fc-shortcuts-overlay'),
+    shortcutsClose: document.getElementById('fc-shortcuts-close'),
     btnUndo: document.getElementById('fc-btn-undo'),
     btnRedo: document.getElementById('fc-btn-redo'),
     historyCount: document.getElementById('fc-history-count'),
@@ -2069,6 +2073,24 @@
       });
     }
 
+    // v4.13.0 — Keyboard shortcuts overlay. Triggered by the toolbar
+    // button or the ? key. Esc closes. Wired as a separate modal (not
+    // via promptInline / confirmInline) because those are single-input
+    // forms; this one is a read-only table.
+    function showShortcutsOverlay() {
+      if (els.shortcutsModal) els.shortcutsModal.hidden = false;
+      if (els.shortcutsOverlay) els.shortcutsOverlay.hidden = false;
+    }
+    function hideShortcutsOverlay() {
+      if (els.shortcutsModal) els.shortcutsModal.hidden = true;
+      if (els.shortcutsOverlay) els.shortcutsOverlay.hidden = true;
+    }
+    if (els.btnHelp) els.btnHelp.addEventListener('click', showShortcutsOverlay);
+    if (els.shortcutsClose) els.shortcutsClose.addEventListener('click', hideShortcutsOverlay);
+    if (els.shortcutsOverlay) {
+      els.shortcutsOverlay.addEventListener('click', hideShortcutsOverlay);
+    }
+
     // v4.13.0 — Open from .mmd/.md file. Pops a system Open dialog,
     // strips the ```mermaid fence (if any), parses via fromMermaid()
     // (inlined above) and replaces the current graph. Confirms
@@ -2150,10 +2172,29 @@
     // Keyboard shortcuts — Ctrl/Cmd+Z / Ctrl/Cmd+Shift+Z. v4.13.0 — added
     // copy/paste/duplicate (Cmd+C / Cmd+V / Cmd+D) plus Delete/Backspace
     // for the canvas-owned selection (replaces the old DOM .selected read).
+    // v4.13.0 — ? opens the shortcuts overlay; Esc closes it.
     let _clipboard = null; // module-level clipboard for Cmd+C/V/D
     document.addEventListener('keydown', (ev) => {
+      // Esc closes the shortcuts overlay regardless of meta state.
+      if (ev.key === 'Escape' && els.shortcutsModal && !els.shortcutsModal.hidden) {
+        hideShortcutsOverlay();
+        ev.preventDefault();
+        return;
+      }
+      // ? toggles the overlay. Use Shift+/ since ? requires Shift on US
+      // keyboards; this catches both the literal character and the key.
+      const isMeta = ev.ctrlKey || ev.metaKey;
+      if ((ev.key === '?' || (ev.key === '/' && ev.shiftKey)) && !isMeta && !ev.altKey) {
+        if (els.shortcutsModal && els.shortcutsModal.hidden) {
+          showShortcutsOverlay();
+        } else {
+          hideShortcutsOverlay();
+        }
+        ev.preventDefault();
+        return;
+      }
       if (!_store) return;
-      const meta = ev.ctrlKey || ev.metaKey;
+      const meta = isMeta;
       if (meta && !ev.shiftKey && ev.key.toLowerCase() === 'z') {
         ev.preventDefault();
         _store.undo();