Markdown: preview opens to the side when being opened from tab context menu

[bpasero]

Refs: #7945

I would expect it to open on the same side unless I press the modifier key and click (note that support for detecting the pressed key for context menus is coming in future versions of electron).

[jrieken]

@bpasero Any other markdown'ish editor that you open or does this happen when nothing is open?

[bpasero]

Just one open:

[jrieken]

yeah, should not be like that...