To reproduce:
1. Open any new project
2. Drag the mouse cursor to the edge of any Unity window
3. When drag cursor appears drag it a bit lower
4. Try to resize the window

Expected result: the window is resized
Actual result: the window is not resized

Reproduced on:
2018.1.0a3 2018.1.0b3 2018.1.0f2 2018.2.0a1

Not reproduced on:
2017.3.2f1 2018.1.0a2 2018.2.0b6

Notes:
Reproduces on win10.
This is an annoying issue for the user as he is resizing windows very often and it doesn't work every time even though the resize cursor is shown.