feat(mermaid): add basic support for vertical layout

src/public/app/widgets/floating_buttons/switch_layout_button.ts

@@ -0,0 +1,38 @@
+import options from "../../services/options.js";
+import NoteContextAwareWidget from "../note_context_aware_widget.js";
+
+const TPL = `
+<button type="button"
+    class="switch-layout-button"
+    title="Switch layout">
+    <span class="bx bxs-dock-bottom"></span>
+</button>
+`;
+
+export default class SwitchSplitOrientationButton extends NoteContextAwareWidget {
+    isEnabled() {
+        return super.isEnabled()
+            && ["mermaid"].includes(this.note?.type ?? "")
+            && this.note?.isContentAvailable()
+            && this.noteContext?.viewScope?.viewMode === "default";
+    }
+
+    doRender(): void {
+        super.doRender();
+
+        this.$widget = $(TPL);
+        this.$widget.on("click", () => {
+            const currentOrientation = options.get("splitEditorOrientation");
+            options.save("splitEditorOrientation", toggleOrientation(currentOrientation));
+        });
+        this.contentSized();
+    }
+}
+
+function toggleOrientation(orientation: string) {
+    if (orientation === "horizontal") {
+        return "vertical";
+    } else {
+        return "horizontal";
+    }
+}