Reproduction Steps:
1. Open Project "Test017"
2. Open test scene
3. Look at the Game View window and notice that there are text fields
4. Notice that Canvas UI Scale Mode is "Scale With Screen Size"
4. Resize the Game View window (hight)
5. Notice that the amount of rows of text in the TextFields differs depending on the scale

Expected Behaviour: Resizing the Canvas Text field and text inside scales equally
Actual Results: If Text Field is just about the right size you can notice that text and its field does not scale equally

Reproduced on versions: Unity 5.3.4f1, 5.3.6p4, 5.4.0f3, 5.4.0p4, 5.5.0b2

Note: Text Vertical Overflow is set Truncate to make the issue visible. If text line disappears it means that UI Text and text inside it was not scaled evenly.