When adding a gallery to post via native WP MediaLibrary on the second modal window (Gallery Settings) it is possible to add caption below chosen media. If you want to do this on mobile (iPhone, iPad) or in mobile simulator in browser (Opera) the input text field became inactive and even not focused.
All other text input fields on this modal window work correctly.
I test this issue on 3 different web sites with different themes and plugins installed. So I guess that is the WP core problem not theme or any plugin.
I’ve already asked this question on the WP forum (no answer yet), but may be someone already stacked with this and have a solution.
Read more here: WordPress – Input caption in gallery does not work on mobile