QGIS » QGIS Application

In QGIS 3.2 this is still an issue.
Resizing the Layer Properties window to the minimum vertical size still makes the histogram collapse into something that is not recognisable as a histogram in the "Sybmology" tab when the "Layer rendering" part is opened.
If the "Layer rendering" part is collapsed, the histogram is visible even when the vertical size is minimal.

Please test with QGIS 3.4 - QGIS 2.18 reached it's end of life.

Tested with 3.4.3.
The histogram is visible (improvement from QGIS 2), but still too small when "Layer Rendering" is expanded and the window has the minimum vertical size.

End of life notice: QGIS 2.18 LTR

Source:
http://blog.qgis.org/2019/03/09/end-of-life-notice-qgis-2-18-ltr/

QGIS 3.4 has recently become our new Long Term Release (LTR) version. This is a major step in our history – a long term release version based on the massive updates, library upgrades and improvements that we carried out in the course of the 2.x to 3x upgrade cycle.

We strongly encourage all users who are currently using QGIS 2.18 LTR as their preferred QGIS release to migrate to QGIS 3.4. This new LTR version will receive regular bugfixes for at least one year. It also includes hundreds of new functions, usability improvements, bugfixes, and other goodies. See the relevant changelogs for a good sampling of all the new features that have gone into version 3.4

Most plugins have been either migrated or incorporated into the core QGIS code base.

We strongly discourage the continued use of QGIS 2.18 LTR as it is now officially unsupported, which means we’ll not provide any bug fix releases for it.

You should also note that we intend to close all bug tickets referring to the now obsolete LTR version. Original reporters will receive a notification of the ticket closure and are encouraged to check whether the issue persists in the new LTR, in which case they should reopen the ticket.

If you would like to better understand the QGIS release roadmap, check out our roadmap page! It outlines the schedule for upcoming releases and will help you plan your deployment of QGIS into an operational environment.

The development of QGIS 3.4 LTR has been made possible by the work of hundreds of volunteers, by the investments of companies, professionals, and administrations, and by continuous donations and financial support from many of you. We sincerely thank you all and encourage you to collaborate and support the project even more, for the long term improvement and sustainability of the QGIS project.

As commented earlier, the issue persists in 3.4 (even if there has been a slight improvement).
The minimum height of the histogram component should be increased, so that it is can be recognized as a histogram.