src/xdisp.c: Enhance commentary.

src/xdisp.c

@@ -98,7 +98,9 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 
       This function attempts to redisplay a window by reusing parts of
       its existing display.  It finds and reuses the part that was not
-      changed, and redraws the rest.
+      changed, and redraws the rest.  (The "id" part in the function's
+      name stands for "insert/delete", not for "identification" or
+      somesuch.)
 
     . try_window
 
@@ -113,6 +115,19 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
    optimizations were successful, redisplay calls redisplay_windows,
    which performs a full redisplay of all windows.
 
+   Note that there's one more important optimization up Emacs's
+   sleeve, but it is related to actually redrawing the potentially
+   changed portions of the window/frame, not to reproducing the
+   desired matrices of those potentially changed portions.  Namely,
+   the function update_frame and its subroutines, which you will find
+   in dispnew.c, compare the desired matrices with the current
+   matrices, and only redraw the portions that changed.  So it could
+   happen that the functions in this file for some reason decide that
+   the entire desired matrix needs to be regenerated from scratch, and
+   still only parts of the Emacs display, or even nothing at all, will
+   be actually delivered to the glass, because update_frame has found
+   that the new and the old screen contents are similar or identical.
+
    Desired matrices.
 
    Desired matrices are always built per Emacs window.  The function
@@ -15757,7 +15772,51 @@ set_vertical_scroll_bar (struct window *w)
    selected_window is redisplayed.
 
    We can return without actually redisplaying the window if fonts has been
-   changed on window's frame.  In that case, redisplay_internal will retry.  */
+   changed on window's frame.  In that case, redisplay_internal will retry.
+
+   As one of the important parts of redisplaying a window, we need to
+   decide whether the previous window-start position (stored in the
+   window's w->start marker position) is still valid, and if it isn't,
+   recompute it.  Some details about that:
+
+    . The previous window-start could be in a continuation line, in
+      which case we need to recompute it when the window width
+      changes.  See compute_window_start_on_continuation_line and its
+      call below.
+
+    . The text that changed since last redisplay could include the
+      previous window-start position.  In that case, we try to salvage
+      what we can from the current glyph matrix by calling
+      try_scrolling, which see.
+
+    . Some Emacs command could force us to use a specific window-start
+      position by setting the window's force_start flag, or gently
+      propose doing that by setting the window's optional_new_start
+      flag.  In these cases, we try using the specified start point if
+      that succeeds (i.e. the window desired matrix is successfully
+      recomputed, and point location is within the window).  In case
+      of optional_new_start, we first check if the specified start
+      position is feasible, i.e. if it will allow point to be
+      displayed in the window.  If using the specified start point
+      fails, e.g., if new fonts are needed to be loaded, we abort the
+      redisplay cycle and leave it up to the next cycle to figure out
+      things.
+
+    . Note that the window's force_start flag is sometimes set by
+      redisplay itself, when it decides that the previous window start
+      point is fine and should be kept.  Search for "goto force_start"
+      below to see the details.  Like the values of window-start
+      specified outside of redisply, these internally deduced values
+      are tested for feasibility, and ignored if found to be
+      unfeasible.
+
+    . Note that the function try_window, used to completely redisplay
+      a window, accepts the window's start point as its argument.
+      This is used several times in the redisplay code to control
+      where the window start will be, according to user options such
+      as scroll-conservatively, and also to ensure the screen line
+      showing point will be fully (as opposed to partially) visible on
+      display.  */
 
 static void
 redisplay_window (Lisp_Object window, bool just_this_one_p)
@@ -15803,6 +15862,8 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
   eassert (XMARKER (w->start)->buffer == buffer);
   eassert (XMARKER (w->pointm)->buffer == buffer);
 
+  /* We come here again if we need to run window-text-change-functions
+     below.  */
  restart:
   reconsider_clip_changes (w);
   frame_line_height = default_line_pixel_height (w);
@@ -15867,7 +15928,7 @@ redisplay_window (Lisp_Object window, bool just_this_one_p)
        && !current_buffer->prevent_redisplay_optimizations_p
        && !window_outdated (w));
 
-  /* Run the window-bottom-change-functions
+  /* Run the window-text-change-functions
      if it is possible that the text on the screen has changed
      (either due to modification of the text, or any other reason).  */
   if (!current_matrix_up_to_date_p