Steps To Reproduce

1. Go Responsive Design Mode
2. Resize the viewport size so that main browser has vertical scrollbar

Actual Results:
Responsive Design Mode UI toolbar covers scrollbar arrow .
See attached screenshot

Expected Results:
Should not cover

Regression window:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=7516aaa595a0c9c7223e4281185233ca6b87a980&tochange=5f7ee717b8528804347cbb2e877865a3e34d6991

Because this bug's Severity has not been changed from the default since it was filed, and it's Priority is P3 (Backlog,) indicating it has been triaged, the bug's Severity is being updated to S3 (normal.)

It looks like one way to fix this is to insert the rdmFrame (an iframe) as a prepended sibling to the browser stack, as opposed to the first child of it (with a z-index that layers it on top of the browser itself). That raises the challenge that we have to calculate a specific height for the rdmFrame, which currently is coded as a css variable in responsive/index.css. That css file isn't loaded until the rdmFrame source is set. Tricky.

I'll see if I can uplevel the variable, and get functions like this.browserWindow.ResizeObserver to make the correct calculated adjustments to the rdmFrame height.