fix(ui): paint composer caret on first padding click (#2509)

packages/ui/src/components/chat/composer/editor/hostMouseDown.ts

@@ -0,0 +1,25 @@
+import type { EditorView } from '@codemirror/view';
+import type { MouseEvent } from 'react';
+
+type ComposerHostMouseDownEvent = Pick<
+    MouseEvent,
+    'target' | 'clientX' | 'clientY' | 'preventDefault'
+>;
+
+export function handleComposerHostMouseDown(
+    view: EditorView | null,
+    event: ComposerHostMouseDownEvent,
+): void {
+    if (!view || view.state.readOnly || !view.contentDOM.isContentEditable) return;
+    // A click that already landed in the text needs no help, and
+    // forwarding it would break drag-selection.
+    if (view.contentDOM.contains(event.target as Node)) return;
+
+    event.preventDefault();
+    const position = view.posAtCoords({ x: event.clientX, y: event.clientY })
+        ?? view.state.doc.length;
+    // Focus before dispatching so CodeMirror updates the drawn caret's
+    // visibility while applying the selection update.
+    view.focus();
+    view.dispatch({ selection: { anchor: position } });
+}