I was able to recreate this and did some additional testing to isolate the problem as best as I could. It seems to be an issue during the hiding/revealing of the vertical tabs menu.
Steps to reproduce:
- Ensure "Toggle Vertical Tabs" option is enabled in the View menu
- Open an Orion window and navigate to any site, e.g. http://example.com/
- Hold the
Command-⌘ key and quickly press the
U key twice (to activate and deactivate Focus Mode quickly)
This causes the width of what looks like the viewport to shrink, but I cannot confirm it is the viewport as opening the dev tools fixes the issue and having the dev tools open prevents the issue from happening at all.
Also, depending on the the initial state of Orion this can either grow or shrink the viewport. If Focus Mode is initially disabled, the viewport will shrink. If Focus Mode is initially enabled, the viewport will grow.
If I had to guess, it is caused by Focus mode being disabled during the shrink animation of the Vertical Tab bar. This makes the tab bar instantly reappear, but the viewport size is still adjusted along the entire animation.