1. Logout from the Backend
2. Go to the login mask /typo3/
3. Insert username & password
4. Press the Login Button and directly change your browsertab now
5. Wait a few seconds (4-5 seconds to be sure) and tab back to the TYPO3 Backend

You now have the same view you can see on the original screenshot 1 here. The right frame could not be loaded.
If you now click on "Page" - for example - the module is loaded but not in a frame, same behaviour as you can see on screenshot 2.

Another - to me - relevant scenario¶

1. I logged in into dev-master with Google Chrome Version 57.0.2987.98 (64-bit) (Mac) as admin (where no 3rd party ext except my theme ext is active which don't add any backend js or css)
2. Clicked on EM
3. Extension Manager was then opened like target="_top"

(This was not reproducable; Was first visit with that browser on that TYPO3 instance; 150/30 Inet connection)

Is this still reproducible with current master? I tried it with Chrome and Firefox on windows and even with throttling on there was no issue.

I can still reproduce that the same way I wrote with the current master.
If you can't reproduce it that way, try to sign in / sign out a few times and then follow these steps.

Let me know if you need further informations.

TYPO3 8.7.1 stable still affected

I can reproduce the issue all the time with TYPO3 8.7.2. Any patch available for this issue?

Reproduce issue:
Chromium > Developer Tools > Network > GPRS throttling (50kb/s) + open record with RTE

No issue:
Chromium > Developer Tools > Network > Disable cache

Solution:
BE compression level = 9

It seems disk cache issue in Chromium. JS compression helps, but still not a solution for a low internet speed.

@Bessei: Please update to v8.7.3 first. You lack some 200+ bugfixes in 8.7.1.

Reproducible in TYPO3 8.7.3. The backend is broken and also the sys_category tree not loading sometimes. I think all these issues are related.

Same problem for me with TYPO3 CMS 8.7.8 : only top bar and left bar are displayed when I'm login.
I'm also with a dynamic IP.
Same problem when I want to log in the install tool.

Removing this cookie "be_lastLoginProvider" seems to solve the problem with Firefox.

Experienced the exact same issue with broken backend in TYPO3 8.7.7 after re-login (only left pane is visible).
The only way to fix this is to clear the browsers cache before re-login.
Can reproduce on Firefox 57.0 and Chrome 62.0

This is what I get in the debug Console:

TypeError: $(...)[0] is undefined    Viewport.js:127:15

<anonymous>  https://mydomain/typo3/sysext/backend/Resources/Public/JavaScript/Viewport.js:127:15
execCb       https://mydomain/typo3temp/assets/compressed/require-684ca2d77dd313b71087737baa0681c7.js:1:12895
check        https://mydomain/typo3temp/assets/compressed/require-684ca2d77dd313b71087737baa0681c7.js:1:6631
enable       https://mydomain/typo3temp/assets/compressed/require-684ca2d77dd313b71087737baa0681c7.js:1:9376
init         https://mydomain/typo3temp/assets/compressed/require-684ca2d77dd313b71087737baa0681c7.js:1:5733
o            https://mydomain/typo3temp/assets/compressed/require-684ca2d77dd313b71087737baa0681c7.js:1:4285
completeLoad https://mydomain/typo3temp/assets/compressed/require-684ca2d77dd313b71087737baa0681c7.js:1:12126
onScriptLoad https://mydomain/typo3temp/assets/compressed/require-684ca2d77dd313b71087737baa0681c7.js:1:13050

It appears that the problem was transient. Can this be related to a slight latency we had on the network earlier? Does this have anything to do with the compression level?

When using firefox-esr 52.6.0esr-1~deb9u1 this happens to me as well:

TypeError: TYPO3.Components.PageTree.DataProvider is undefined
TYPO3.Components.PageTree.App<.initComponent
Ext.Component
.extend</&lt;/T&lt;
.extend</&lt;/T&lt;
.extend</&lt;/T&lt;
.extend</&lt;/T&lt;
TYPO3.ModuleMenu.App.loadNavigationComponent
TYPO3.ModuleMenu.App.loadModuleComponents
TYPO3.ModuleMenu.App.showModule
TYPO3.ModuleMenu.App.initialize
&lt;anonym&gt;
newContext/u.execCb
newContext/t.prototype.check
newContext/t.prototype.enable/</<
bind/<
newContext/t.prototype.emit/<
each
newContext/t.prototype.emit
newContext/t.prototype.check
newContext/t.prototype.enable
newContext/t.prototype.init
o
newContext/u.completeLoad
newContext/u.onScriptLoad

But with a clean profile

firefox -ProfileManager -no-remote

I did not have this error.
So one remedy was disabling all extensions in the original profile.
Also whitelisting uMatrix completely did not help here.

Patch set 1 for branch TYPO3_8-7 of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/57527

In TYPO3 8.7.16 it's enough to log into the backend and then reload the whole view once (with having browser console collapsed).