Under Window
-> Preferences
go to General
-> Editors
.
The last point should be "Close editors automatically", and you can specify a number.
If you check this, the least recently used editor will be closed when the given amount of opened editors is reached. That way the open editors don't take up so much space, and the possibility of truncated names is reduced.
You can always quickly search and open resources with Ctrl+Shift+R, so there is no need to have them open all the time.
Of course it can't be guaranteed to always show the complete name. If there is not enough space, what do you expect Eclipse to do?