Fix bug #17892 with mode/header line and display margins.

 src/dispnew.c (prepare_desired_row): Accept 2 additional arguments:
 the window whose glyph row is being prepared and a flag whether it
 is for mode/header line.  Make sure the glyph row's marginal areas
 are in sync with what the window wants.
 src/xdisp.c (display_line, display_mode_line): Call
 prepare_desired_row with additional arguments, as appropriate.
 src/dispextern.h (prepare_desired_row): Adjust prototype.
 src/window.h: Improve commentary of the marginal columns.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/window.h b/src/window.h
index bdc5dddb7e5..b9c2b1f5ba8 100644
--- a/src/window.h
+++ b/src/window.h
@@ -271,8 +271,10 @@ struct window
     int left_fringe_width;
     int right_fringe_width;
 
-    /* Width of left and right marginal areas in columns.
-       A value of 0 means no margin.  */
+    /* Requested width of left and right marginal areas in columns.  A
+       value of 0 means no margin.  The actual values are recorded in
+       the window's glyph matrix, in the left_margin_glyphs and
+       right_margin_glyphs members.  */
     int left_margin_cols;
     int right_margin_cols;