Recreation steps:
1.Open Unity
2.Create a new scene
3.Add and Input field
4.Change the width of Rect Transform to 900 and the height to 300
5.Change the text to Best Fit under the paragraph section in the Text (script) component
6.Under Input Field (Script) component change Line type to Multi Line Newline
7 enter AAA
AAAAAAAAAAAAAA
aaaaa
aa
aaaaa
a
a
aaaaaaAAAAAAAAA
aaaAAAAAA
aaa
a
a
a
aAAA

aAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAA

aaaa
aAa

8. You will stop seeing any more letters as you type.

Actual vs Expected:
Expected: text scales to the textField size,
Actual: text disappears

Reproduced on Unity 5.3.4p6, 5.3.3f1, 5.2.4f1, 5.1.1f1.

in 5.1.1f1 it also throws an error
"String too long for TextMeshGenerator. Cutting off characters.
UnityEngine.EventSystems.EventSystem:Update()"