FIX: Allow touchmove in composer when there is a selection (#30957)

We stop propagating the `touchmove` event in the composer to ensure that
the page doesn't scroll inadvertently. We should only do this if no text
is selected, otherwise we block making changes to the text selection on
the `textarea` element.

app/assets/javascripts/discourse/app/lib/composer/composer-position.js

@@ -13,7 +13,9 @@ export function setupComposerPosition(editor) {
     // when the editor does not have any content to scroll
     applyBehaviorTransformer("composer-position:editor-touch-move", () => {
       const notScrollable = editor.scrollHeight <= editor.clientHeight;
-      if (notScrollable) {
+      const selection = window.getSelection();
+
+      if (notScrollable && selection.rangeCount === 0) {
         event.preventDefault();
         event.stopPropagation();
       }