Added missing la_fld_ExtraHTTPHeaders phrase.

All test plan passed.

I've added # (new) apply the patch from the D524, unless it's already committed line, that should fix the problem. Please retest.

2.2 - this part of test plan is excessive

2.2 - this part of test plan is excessive
2.3 - Failed all front-end tests. Always shown Oops, something went wrong text, not 404 page. Always System Log IS created about incident

Added code to support changes from the D466.

1. resubmit from the 5.3.x branch, because an associated Jira task is for the 5.3.x branch as well.
2. (test plan only tests 50% of changed code) improve test plan by checking, that request header logging still works, when HttpQuery is present

In D519#10607, @erik wrote:

Part 2 - "confirm, that you're seeing an Admin Console" - no, adding test code to OnLogin prevents from login to the adm.console.

Boyscout improvements in session.php

Part 2 - "confirm, that you're seeing an Admin Console" - no, adding test code to OnLogin prevents from login to the adm.console.
Part 3 - "replace the tab URL with the provided URL (text after #redirect#)" - seems, missing some action and/or confirmation after this step ot the test plan

Applying the Boyscout Rule principe.

In D518#10578, @erik wrote:

Today I have z-index; 0; in DIV Can't provide screenshot without -index today.

Today I have z-index; 0; in DIV Can't provide screenshot without -index today.

In D518#10573, @erik wrote:

Part 2 - all right, so revision is accepted.

Added reference to the used code origins.

done all requested code improvements

Update test plan to cover requested changes as well.

In D513#10483, @erik wrote:

Part 3.1.1 login to the Admin Console by accessing it on the sd1.test.domain.com domain over http:// protocol - FAILED, after login attempt - redirect to the login page. SUCCESS after cookies deleted (beyond test plan)
Part 3.1.12 confirm, that orange buttons for editing page/content blocks are now shown - NOT STABLE. At first attempt - no orange buttons. Then I logged in from main domain, set Default theme as primary theme, checked that orange buttons shown, restored Advanced theme as primary theme, then tried again login to sd1 domain, failed, cleared cookies, login to sd1 admin - and only then orange buttons shown

In D511#10453, @erik wrote:

Part 3.2 go to the Configuration → Website → Advanced section - FAILED, can't. login, redirects to login screen. Logged in only after all current site cookies deleted before login.

Part 3.1.1 login to the Admin Console by accessing it on the sd1.test.domain.com domain over http:// protocol - FAILED, after login attempt - redirect to the login page. SUCCESS after cookies deleted (beyond test plan)
Part 3.1.12 confirm, that orange buttons for editing page/content blocks are now shown - NOT STABLE. At first attempt - no orange buttons. Then I logged in from main domain, set Default theme as primary theme, checked that orange buttons shown, restored Advanced theme as primary theme, then tried again login to sd1 domain, failed, cleared cookies, login to sd1 admin - and only then orange buttons shown

Added some explanatory comments.

In D513#10467, @erik wrote:

Part 3.1.7 confirm that [SD ID: 1] is added in the current page blur bar (the 1 is site domain ID) - NO, [SD ID: ] is added. And this is right, because at Preparations 10 "add 1st site domain with: Domain Name=sd1-http.test.domain.com", not sd1.test.domain.com
Part 3.1.9 confirm, that you'll see [Site Domain ID: 1] text (the 1 is site domain ID) - NO I see [Site Domain ID: 0]

Part 3.1.7 confirm that [SD ID: 1] is added in the current page blur bar (the 1 is site domain ID) - NO, [SD ID: ] is added. And this is right, because at Preparations 10 "add 1st site domain with: Domain Name=sd1-http.test.domain.com", not sd1.test.domain.com
Part 3.1.9 confirm, that you'll see [Site Domain ID: 1] text (the 1 is site domain ID) - NO I see [Site Domain ID: 0]
Part 3.1.12 logout - FAILED, infinite redirect to http://sd1.test.domain.com/SVN/5.3.x/admin/index.php?env=-index%3Am0--default---&expired=1 happens
Part 3.2.2 confirm that you're looking at Admin Console at the test.domain.com domain over https:// protocol - NO, I'm looking at Admin Console at the sd1.test.domain.com domain over https:// protocol
Part 3.2.3 confirm that [SD ID: 1] is added in the current page blur bar (the 1 is site domain ID) = NO, [SD ID: ] is added in the current page blue bar
Part 3.2.5 confirm, that you'll see [Site Domain ID: 1] text (the 1 is site domain ID) - NO I see [Site Domain ID: 0]
Part 4.2 confirm, that you'll see [Site Domain ID: 1] text (the 1 is site domain ID) - NO I see [Site Domain ID: 0]
Part 4.4 confirm, that you'll see [Site Domain ID: 1] text (the 1 is site domain ID) - NO I see [Site Domain ID: 0]