According to the previous change, `.half` position meaning is
also changed so that the height calc logic must be modified.
Previously, for example, `.half` position has 2 meaning. One is
a position at half of a screen. Another is a position when a
content is visible half. `.tip` position is also same.
Now `.half`/`.tip` position can display a full content if it's
top most. In the case, the surface height should fit to a height
for `.half`/`.tip` position.
Shin Yamamoto
a15444d237