Steps to reproduce:

1) Download attached project "Bug_GUILabel.zip" and open in Unity.
2) In Toolbar, click on Tools >> WindowBugEditorLabel to open WindowTests.
Note: Make sure the windows width way much smaller than the table width.
3) Grab handle of the Horizontal slider and try to drag left and right.
Note: Handle of horizontal slider will stop randomly. (see attached gif "stop.gif").
4) Press the button "Using EditorGUI.labelField" in the top left corner. It replaces using EditorGUI.LabelField to GUI.Label.
5) Try to move handle of the Horizontal slider.
Note : It works as expected.(see attached gif "works.gif").
6) Press button again to replace GUI.Label to EditorGUI.LabelField.
7) Increase size of the WindowTest to the last Column.
Note: Horizontal slider will work as expected.(see attached gif "increased.gif").

Reproduced with: 5.3.4f1, 5.3.5p6, 5.4.0b24.