Scale bar missing in https://github.com/GNOME/epiphany browser.
https://github.com/openstreetmap/iD/issues/5435
Visible in chromium, missing in epiphany.
https://en.wikipedia.org/wiki/GNOME_Web
https://wiki.gnome.org/Apps/Web
Cannot reproduce, scale bar is visible in epiphany here. Seems like some other issue on your machine.
iD does have a scale bar. It does go away if your window really is too small.
I've included a screen shot, but you can also search the issue list for "scale bar" to find probably a dozen more screenshots and descriptions of this feature.

Ah ha!
Observe, at CTRL+0 zoom, scale bar present.

Now at CTRL++ zoom,

Poof! scale bar gone!
Please only make it go away when it finally bangs into something.
Wow you must be on a tiny screen - what is the resolution?
@tastrax Default resolution for IBM Thinkpad 50e, 1024x768 I recall... (but I'm on a different terminal today.)
Now testing with Chromium at 1152x864. Scale bar remains until CTRL++++
zoom (150%) whereupon it disappears. But there is still plenty of room for it:

Why not just use the + key to zoom in? Why do you need everything magnified?
Why not just use the + key to zoom in? Why do you need everything magnified?
Usually because the fonts are too small, etc. (age 57).
I鈥檒l look into it , but your browser may be trying to zoom the scale bar itself like any other graphic element . A zoomed-in scale bar will obviously be wrong and lie to you.
The scale bar does also disappear in Edge and Firefox. It doesn't seem to become lying as long as it is visible.
Most helpful comment
I鈥檒l look into it , but your browser may be trying to zoom the scale bar itself like any other graphic element . A zoomed-in scale bar will obviously be wrong and lie to you.