Merge branch 'ux-guide-dropdown-max-height' into 'master'

UX Guide: add guidance for max height for dropdowns

Added guidance to the Components page of the UX Guide, documenting the recommend max height for a dropdown.

See merge request !7766

doc/development/ux_guide/components.md

@@ -91,7 +91,9 @@ TODO: Will update this section when the new filters UI is implemented.
 
 ![Dropdown states](img/components-dropdown.png)
 
+### Max size
 
+The max height for dropdowns should target **10-15 items**. If the height of the dropdown is too large, the list becomes very hard to parse and it is easy to visually lose track of the item you are looking for. Usability also suffers as more mouse movement is required, and you have a larger area in which you hijack the scroll away from the page level. While it may initially seem counterintuitive to not show as many items as you can, it is actually quicker and easier to process the information when it is cropped at a reasonable height.
 
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