Subject: 26.3; format-mode-line with FACE argument

From: [email-masked]
Date: Sat, 25 Jan 2020 00:05:25 +0900

#'format-mode-line assumes all characters of a string have a same 'face
property value and checks only the first character.

  (format-mode-line (concat "A" (propertize "XYZ" 'face 'region)) 'highlight)
  => #("AXYZ" 0 1 (face highlight) 1 4 (face highlight))

#'format-mode-line merges the 'face property value of the first
character of a string with FACE argument. It is not ducumented at all.

  (format-mode-line (propertize "ABC" 'face 'region) 'highlight)
  => #("ABC" 0 3 (face (region highlight)))

#'format-mode-line called with FACE=t should use 'mode-line or
'mode-line-inactive instead of 't, but it treats 't as a face.

  (format-mode-line "ABC" t)
  => #("ABC" 0 3 (face t))

In GNU Emacs 26.3 (build 1, x86_64-w64-mingw32)
 of 2019-08-29 built on CIRROCUMULUS
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