[Affected versions]
Firefox 76.0a1

[Affected platforms]
All

[Steps to reproduce]

1. Open the Firefox browser with a new profile and open a new tab.
2. Hit Ctrl+Shift+M in order to start RDM.
3. Type a different address in the URL bar and hit ENTER.

[Expected result]
Responsive Design mode should remain open.

[Actual result]
Responsive Design mode closes when its loading the website.

Rares, are you able to reproduce this with old-RDM? Navigating between different domains will close RDM in both versions. We recently implemented target-switching, which causes RDM to close during a process switch.

The bug that introduces this issue started in Bug 1603370. You need to have devtools.target-switching.enabled set to true for RDM to remain open when navigating between different domains.

(In reply to Micah Tigley [:mtigley] from comment #1)

Navigating between different domains will close RDM in both versions. We recently implemented target-switching, which causes RDM to close during a process switch.

This is incorrect, my apologies. RDM will close for new-RDM if target-switching is not enabled. Old-RDM will actually fire fake remoteness events, which is why it will stay open when navigating between different domains. (for own reference, see: https://searchfox.org/mozilla-central/source/devtools/client/responsive/ui.js#1226)

Example websites that will trigger this issue: https://www.mozilla.org -> https://searchfox.org/

(In reply to Micah Tigley [:mtigley] from comment #2)

Example websites that will trigger this issue: https://www.mozilla.org -> https://searchfox.org/

I can only reproduce this with a transition from an about: page to an external domain. For example about:home -> https://www.mozilla.org.

(In reply to Brad Werth [:bradwerth] from comment #3)

I can only reproduce this with a transition from an about: page to an external domain. For example about:home -> https://www.mozilla.org.

You might have to make sure fission.autostart is also set to true.

I have tried to set the pref before creating the profile and with devtools.target-switching.enabled = true this issue does not occur, the weird part is that if you start an older profile and not a completely new one this issue does not occur.
I've also set fission.autostart and I cant reproduce it.

I also can't reproduce the issue if I start a fresh new profiles with the devtools.responsive.browserUI.enabled = false.

Another thing we noticed that if you have RDM on on any website and click the home page button it will exit RDM as well.
Is this a separate issue ? we noticed that this issue occurs with devtools.target-switching.enabled = false as well.
Should we log a new issue for it ?

(In reply to Rares Doghi from comment #6)

Another thing we noticed that if you have RDM on on any website and click the home page button it will exit RDM as well.
Is this a separate issue ? we noticed that this issue occurs with devtools.target-switching.enabled = false as well.
Should we log a new issue for it ?

Rares, as we discussed on Slack I believe this issue is the same since the homepage default is an about: page. The issue goes away if the homepage is set to the same external domain. So I think it's fine to keep this issue here.

Backed out changeset 77a5673f90f4 (Bug 1625501) for causing devtools failures at devtools/client/responsive/test/browser/browser_container_tab.js

Push with failure: https://treeherder.mozilla.org/#/jobs?repo=autoland&revision=77a5673f90f4f7aa1ac2d467f3ce1fdf7ff8bebd

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=297007368&repo=autoland&lineNumber=3738

Backout link: https://treeherder.mozilla.org/#/jobs?repo=autoland&selectedJob=297010945&resultStatus=testfailed%2Cbusted%2Cexception&classifiedState=unclassified&revision=b76975bbed0ebb8ff7d35486966247484517c610

[task 2020-04-09T20:35:29.418Z] 20:35:29     INFO - TEST-INFO | started process screentopng
[task 2020-04-09T20:35:29.683Z] 20:35:29     INFO - TEST-INFO | screentopng: exit 0
[task 2020-04-09T20:35:29.683Z] 20:35:29     INFO - TEST-UNEXPECTED-FAIL | devtools/client/responsive/test/browser/browser_container_tab.js | Test timed out - 
[task 2020-04-09T20:35:29.683Z] 20:35:29     INFO - Removing tab.
[task 2020-04-09T20:35:29.683Z] 20:35:29     INFO - Waiting for event: 'TabClose' on [object XULElement].
[task 2020-04-09T20:35:29.684Z] 20:35:29     INFO - Got event: 'TabClose' on [object XULElement].
[task 2020-04-09T20:35:29.684Z] 20:35:29     INFO - Tab removed and finished closing
[task 2020-04-09T20:35:29.685Z] 20:35:29     INFO - TEST-PASS | devtools/client/responsive/test/browser/browser_container_tab.js | The main process DevToolsServer has no pending connection when the test ends - 
[task 2020-04-09T20:35:29.686Z] 20:35:29     INFO - GECKO(3236) | MEMORY STAT | vsize 20975088MB | residentFast 1158MB
[task 2020-04-09T20:35:29.686Z] 20:35:29     INFO - TEST-OK | devtools/client/responsive/test/browser/browser_container_tab.js | took 180282ms
[task 2020-04-09T20:35:29.735Z] 20:35:29     INFO - checking window state
[task 2020-04-09T20:35:29.792Z] 20:35:29     INFO - TEST-START | devtools/client/responsive/test/browser/browser_contextmenu_inspect.js
[task 2020-04-09T20:35:29.828Z] 20:35:29     INFO - Entering test bound 
[task 2020-04-09T20:35:29.864Z] 20:35:29     INFO - Adding a new tab with URL: http://example.com/browser/devtools/client/responsive/test/browser/doc_contextmenu_inspect.html
[task 2020-04-09T20:35:30.244Z] 20:35:30     INFO - Tab added and finished loading
[task 2020-04-09T20:35:30.244Z] 20:35:30     INFO - Opening responsive design mode
[task 2020-04-09T20:35:30.341Z] 20:35:30     INFO - Console message: [JavaScript Error: "The character encoding of the HTML document was not declared. The document will render with garbled text in some browser configurations if the document contains characters from outside the US-ASCII range. The character encoding of the page must be declared in the document or in the transfer protocol." {file: "http://example.com/browser/devtools/client/responsive/test/browser/doc_contextmenu_inspect.html" line: 0}]
[task 2020-04-09T20:35:30.712Z] 20:35:30     INFO - GECKO(3236) | console.log: "[DISPATCH] action type:" "CHANGE_DISPLAY_PIXEL_RATIO"
[task 2020-04-09T20:35:30.720Z] 20:35:30     INFO - GECKO(3236) | console.log: "[DISPATCH] action type:" "ADD_VIEWPORT"
[task 2020-04-09T20:35:31.024Z] 20:35:31     INFO - Responsive design mode opened
[task 2020-04-09T20:35:31.024Z] 20:35:31     INFO - GECKO(3236) | console.log: "[DISPATCH] action type:" "LOAD_DEVICE_LIST_START"
[task 2020-04-09T20:35:31.052Z] 20:35:31     INFO - Waiting for state predicate "state => state.viewports.length == 1"
[task 2020-04-09T20:35:31.052Z] 20:35:31     INFO - Found state predicate "state => state.viewports.length == 1"
[task 2020-04-09T20:35:31.054Z] 20:35:31     INFO - Open the responsive design mode and set its size to 500x500 to start
[task 2020-04-09T20:35:31.054Z] 20:35:31     INFO - Current size: 320 x 480, set to: 500 x 500
[task 2020-04-09T20:35:31.056Z] 20:35:31     INFO - Waiting for viewport-resize to 500 x 500
[task 2020-04-09T20:35:31.056Z] 20:35:31     INFO - GECKO(3236) | console.log: "[DISPATCH] action type:" "RESIZE_VIEWPORT"
[task 2020-04-09T20:35:31.077Z] 20:35:31     INFO - Got content-resize to 500 x 500
[task 2020-04-09T20:35:31.077Z] 20:35:31     INFO - Open the inspector, rule-view and select the test node
[task 2020-04-09T20:35:31.077Z] 20:35:31     INFO - Opening the inspector
[task 2020-04-09T20:35:31.078Z] 20:35:31     INFO - Opening the toolbox

https://hg.mozilla.org/mozilla-central/rev/d045422c07e4

This issue is verified as fixed in our latest Nightly build 77.0a1 (2020-04-13) on Windows 10, Ubuntu 18.04 and Mac OS 10.14.

I will update the status flag for this issues.