MAGETWO-95866 Add horizontal scroll if elements extend menu's width

[ptylek]

Description (*)

If a user install extension which adds multiple columns in any submenu (e.g. Marketing tab), then he wouldn't be able to see those which extend submenu's width.
As this container has position: absolute, we are not able to use flex as its children does not participate in flex layout (https://www.w3.org/TR/css-flexbox-1/#abspos-items).
The other option was to use grid, however it is not widely supported in browsers such as IE11 (https://caniuse.com/#feat=css-grid).
Last but not least, in order to provide access for the user to those additional extension's columns, we can add a horizontal scroll which only appears in that case.

Fixed Issues (if relevant)

1. Left Side Back End Menu Design fix #18687: Left Side Back End Menu Design fix

Manual testing scenarios (*)

1. Install the multiple extension which is creating sub menu in Marketing tab
2. Access the admin side to see how it looks like

Questions or comments

Comment: In order to have scroll below container, I dropped container's min-height (using 100%) in favor of submenu's min-height as a variable.

Screenshots:

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)

[m2-assistant]

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[magento-engcom-team]

Hi @krzksz, thank you for the review.
ENGCOM-6464 has been created to process this Pull Request
✳️ @krzksz, could you please add one of the following labels to the Pull Request?

Label	Description
Auto-Tests: Covered	All changes in Pull Request is covered by auto-tests
Auto-Tests: Not Covered	Changes in Pull Request requires coverage by auto-tests
Auto-Tests: Not Required	Changes in Pull Request does not require coverage by auto-tests

[krzksz]

It's purely visual change, tests not required.

[engcom-Alfa]

✔️ QA Passed

Actual Result:

[m2-assistant]

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