Thank you for helping make Firefox better. If you are reporting a defect, please complete the following:

What were you doing?

1. View -> Zoom -> Zoom In
2. View -> Zoom -> Zoom In
3. Tools -> Web Developer -> Responsive Design Mode
4. Close Responsive Design Mode by clicking 'X' on the banner

What happened?

In the URL bar, it still has a "120%" chiclet, but the actual window is no longer "zoomed in" to 120%. It is now at 100%.

What should have happened?

If RDM resets zoom level, then the chiclet should be gone. Preferably, RDM should restore the previous zoom level.

Anything else we should know?

I did this in Desktop Firefox 78.0a1 (2020-05-12) (64-bit) Nightly, Mac and could reproduce it at will on both old and new profiles.

Thanks for reporting this issue! I can confirm this is reproducible in Firefox Nightly 78. I'll see if I can find the regression window for this.

Using mozregression, I was able to narrow the regression to: https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=66899ba0dcbed00349a84e44fcf4717a11ec4162&tochange=be474ea875e8933f1a6cad062f7f90db208976b3

I can take this.

(just so I don't forget, as I ran out of time today)

We can get two calls to enterResponsiveMode() for a tab, via the two
callsites in devtools/client/responsive/ui.js.

Calling enterResponsiveMode() is not idempotent after bug 1634621, as
the second call will lose the zoom information. Prevent this from
happening by making the call a no-op if we're already in responsive
mode.

Do the same in leaveResponsiveMode() for consistency.

Comment on attachment 9154197 [details]
Bug 1638118 - Ignore redundant calls to enterResponsiveMode() / leaveResponsiveMode(). r=mtigley

Beta/Release Uplift Approval Request

• User impact if declined: Opening RDM may confuse the UI and show bogus zoom values.
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: No
• Needs manual test from QE?: Yes
• If yes, steps to reproduce: comment 0
• List of other uplifts needed: none
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Pretty small, trivial-ish patch.
• String changes made/needed: none

https://hg.mozilla.org/mozilla-central/rev/d037817cc193

Comment on attachment 9154197 [details]
Bug 1638118 - Ignore redundant calls to enterResponsiveMode() / leaveResponsiveMode(). r=mtigley

rdm regression fix, approved for 78.0b4

https://hg.mozilla.org/releases/mozilla-beta/rev/528e484c4d7c

Reproduced the initial issue using an old Nightly build from 2020-05-14, verified that using latest Nightly and Firefox 78.0b4 the issue does not reproduce anymore across platforms (macOS 10.15.6 beta, Windows 10 64bit and Ubuntu 18.04 64bit).