"Open preview to the side" is shown twice for markdown files

[sdirix]

Bug Description:

Open markdown editors show the "Open preview to the side" action twice in the toolbar

Steps to Reproduce:

1. Use Theia IDE 1.58.100 or 1.59.1 (preview)
2. See that the "Open preview to the side" action is shown twice. One with keybinding and one without

3. The one without keybinding opens in split preview, while the one with takes over the main area

Additional regression

The one with keybinding does not work in Theia Try Online

However it works locally

Additional regression (2)

When using the action without keybinding it offers a "Open source" action. When clicking it, the source is opened AGAIN, leading to the editor being open twice. If going through the chain again, the subsequent preview and open source actions will focus the newly opened tabs.

Additional Information

• Operating System: Ubuntu 22.04
• Theia Version: >= 1.58.100 (might occur before too, but did not test it)

[msujew]

Related #13895 (I never got to removing it).