(core) Disable drag-and-drop in active form input

Summary:
Drag-and-drop was interfering with text selection in inputs for form
labels and paragraphs.

Test Plan: Manual.

Reviewers: jarek

Reviewed By: jarek

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D4209

app/client/components/Forms/Editor.ts

@@ -123,6 +123,11 @@ export function buildEditor(props: Props, ...args: IDomArgs<HTMLElement>) {
     dom.on('dragstart', (ev) => {
       // Prevent propagation, as we might be in a nested editor.
       ev.stopPropagation();
+      if (props.editMode?.get()) {
+        ev.preventDefault();
+        return;
+      }
+
       ev.dataTransfer?.setData('text/plain', JSON.stringify(box.toJSON()));
       ev.dataTransfer!.dropEffect = "move";
       dragging.set(true);

app/client/components/Forms/Paragraph.ts

@@ -27,6 +27,7 @@ export class ParagraphModel extends BoxModel {
     return buildEditor({
       box: this,
       overlay: this._overlay,
+      editMode,
       content: css.cssMarkdownRendered(
         css.markdown(use => use(text) || '', dom.hide(editMode)),
         dom.maybe(use => !use(text) && !use(editMode), () => cssEmpty('(empty)')),