I have just implemented a new face property in XEmacs 21.5, called "flush". When some text is displayed in a face which has this property set to t (it's a Boolean property), then the face extends until the right border of the window instead of just the end of the actual line of text. The effect is only visible if the face has a non-default background color or pixmap and gives the text segment the appearance of a block instead of being ragged right. In fact (if that rings a bell to you), this is the equivalent of the block value for the HTML display property.

See the attached screenshot for an example. In that particular case, the buffer displays an article in Gnus and the concerned face is mm-uu-extract. You can see two versions of the same buffer, with and without the property set. There are a number of situations in which setting a face to flush is nicer visually. Probably the most obvious case is that of text selection. Below is a list of faces that I'm currently setting to flush. I'll be updating this list as needed. In order to set a face to flush, either use the Custom interface or the set-face-flush-p function directly.

zmacs-region
diff-nonexistent-face
gnus-summary-cancelled[-face]
mm-uu-extract
mmm-default-submode-face
mmm-code-submode-face