Steps to reproduce:

1. Create a new Unity project
2. Dock some windows with left-aligned text to the very left of the Editor, e.g. Hierarchy, Project, Inspector (Default layout works good for this)
3. Resize the window to the right of the previously docked window horizontally (if using 'Default' layout, resize Scene view)
4. Notice the text, tab name and the icons are jittering constantly when resizing is happening (see the attached gif)

Note:
Doesn't happen when switching to OpenGL Editor.
Doesn't happen (or at least it's very hard to see it happening) when the window's left side is moving as well when resizing the right size.

Reproduced with: 5.6.6f2, 2017.1.4f1, 2018.3.0a11