Please describe the problem:
Note this bug originaly reported to  https://bugs.launchpad.net/ubuntu/+source/gtk+2.0/+bug/113721

The sequences
Multi_key U - [capital U, minus) and Multi_Key U _ (capital U, underscore) should produce Latin Capital U with macron Ū. In fact they generate a lower case u with macron ū

Steps to reproduce:
1. use Compose key sequences to enter character U with macron
2. 
3. 


Actual results:
Ū Multi_key minus U Latin capital U with macron
ū Multi_key U underscore  Latin small U with macron *BUG*
ū Multi_key U minus       Latin small U with macron *BUG*
Ū Multi_key underscore U  Latin capital U with macron
ū Multi_key minus u       Latin small U with macron
ū Multi_key u minus       Latin small U with macron
ū Multi_key u underscore  Latin small U with macron
ū Multi_key underscore u  Latin small U with macron

Expected results:

Ū Multi_key U underscore  Latin capital U with macron 
Ū Multi_key U minus       Latin capital U with macron 

Does this happen every time?
yes

Other information:
As best as I can understand the actual code it would appear that the fix would be to change the unicode string to produce the correct characters eg
/gtk/gtkimcontextsimple.c (gtk+ version 2.10.7)

change lines 635 and 639 from 0x016B to 0x016A

Source line 635
  GDK_Multi_key, GDK_U, GDK_minus, 0, 0, 0x016A, /* LATIN_CAPITAL_LETTER_U_WITH_MACRON */

Source line 639
  GDK_Multi_key, GDK_U, GDK_underscore, 0, 0, 0x016A, /* LATIN_CAPITAL_LETTER_U_WITH_MACRON */

This would make the comments in the source, the sense of the Key sequence using capital to generate a capital-with-diacritic and the unicode value all in agreement and coherent with similar codes for other letters in the file.