Content is incomplete upon export

### ⚠️ This issue respects the following points: ⚠️

- [x] This is a **bug**. Not a [question or feature request](https://github.com/gsantner/markor/issues/new/choose).
- [x] The topic is **not** already reported at [Issues](https://github.com/gsantner/markor/issues?q=#js-issues-search). _(I've searched it and checked most recent issues)_.
- [x] Markor **is** up to date. See [Releases](https://github.com/gsantner/markor/tags) for the latest version. Updates are available from [F-Droid](https://f-droid.org/en/packages/net.gsantner.markor/) and GitHub.
- [x] The bug is still present in the latest development version (git master). (Please [download](https://nightly.link/gsantner/markor/workflows/build-android-project/master) and try the test version of Markor, named **Marder**. Don't worry; Markor and Marder appear as completely separate applications. You can install both side-by-side, and Markor settings are not touched. In case the issue is resolved there, you don't need to create a bug report. The change will be part of the next Markor update.)

### Description

When using the preview mode of Markdown version 2.16.xx, exporting content as an image or PDF only captures the visible on-screen area, while off-screen content renders blank. This issue does not exist in version 2.15.2, which exports images and PDFs without any abnormalities. The problem only occurs in the two most recent releases.

### Steps to reproduce

1. Preview
2. Share as picture or pdf
3.
4.

### Information

Android version: 13
Device (phone model): MiPhone
App Version: 2.16.x

### Source

GitHub Releases

### Format / File type

Markdown

### Additional info / Log

```shell
-
```

Start by reproducing Markdown preview export on Android 13 with Markor 2.16.x, comparing image and PDF output against version 2.15.2. Trace the preview-to-share export path; done means off-screen content is rendered in the exported image and PDF, not left blank.

Written by the indexing model from the issue text.