Repro steps:

1. Open provided project
2. Click on "Window" -> "Seek" (If the "Seek" window is not already open)
3. Notice that the top header is squished, but the icons below aren't
4. Open the "SeekWindows" script file located in "Assets" -> "Editor" -> "Seek"
5. Notice that, at line 31-50, the header is drawn first, then the "IconSize" is changed, then the icons are drawn

Expected result: The header is drawn with the default size and icons are drawn with the later set custom "IconSize"
Actual result: The header and the icons are drawn with the custom size even though the "IconSize" was changed after the header was drawn

Reproduced with: 2017.2.0a1
Regression introduced in: 2017.1.0b6
Did not reproduce on: 5.6.1p1

Fixed in: 2017.1.0b10

Note:
Uncommenting like 46 fixes the issue
Added video of repro and photo of it working correctly.