Following the "Minibuffer tray to display current time and date" discussion, it seems that a bug with the `display' property. Consider the following code:
(setq-default minibuffer-line-format
'((:eval (propertize (format-time-string "%Y.%m.%d")
'face
'minibuffer-line-date))
" "
(:eval (propertize (format-time-string "%A")
'face
'minibuffer-line-weekday))
" "
(:eval (propertize (format-time-string "%R")
'face
'minibuffer-line-time))))
It works as expected.
Consider another piece of code:
(setq-default minibuffer-line-format
`((:eval
(let ((string (concat
(propertize (format-time-string "%Y.%m.%d")
'face
'minibuffer-line-date)
" "
(propertize (format-time-string "%A")
'face
'minibuffer-line-weekday)
" "
(propertize (format-time-string "%R")
'face
'minibuffer-line-time))))
(concat (propertize " "
'display
`((space :align-to
(- right
right-fringe
,(length string)))))
string)))))
Alignment works as expected, but faces are messed up. In fact, the default face is used everywhere (which comes from the `display' property), like if subsequent propertizings of date, weekday, and time have never been there.