>> 
> 1. What's the purpose of the following in
> build_frame_matrix_from_leaf_window?  "frame_row->used[TEXT_AREA]" will
> always end up being equal to "window_matrix->matrix_w" after processing
> the right-most window of the row.

I don't understand.  What about the /matrix_x/ + matrix_w below?

> 
> --8<---------------cut here---------------start------------->8---
>       /* Set number of used glyphs in the frame matrix.  Since we fill
>          up with spaces, and visit leaf windows from left to right it
>          can be done simply.  */
>       frame_row->used[TEXT_AREA]
>         = window_matrix->matrix_x + window_matrix->matrix_w;
> --8<---------------cut here---------------end--------------->8---
> 
> 
> 2. From the documentation of make-terminal-frame:
> 
> > Note that changing the size of one terminal frame automatically
> > affects all frames on the same terminal device.
> 
> 
> But why?  Do they share glyph matrices, since only one of them is
> visible at a single time?

How else should other frames behave?  Leave a "hole" on the terminal where nothing is displayed when the TTY window is made larger?  And what if the terminal is made smaller?

> 
> 
> 3. Why clear the desired matrix in build_frame_matrix?  As I understand,
> that would clear the glyph matrices of windows the frame, because window
> desired matrix shares memory with frame desired matrix, and its seems
> like build_frame_matrix is called after window desired matrices are
> built.
> 
> --8<---------------cut here---------------start------------->8---
>   /* Clear all rows in the frame matrix covered by window matrices.
>      Menu bar lines are not covered by windows.  */
>   for (i = FRAME_TOP_MARGIN (f); i < f->desired_matrix->nrows; ++i)
>     clear_glyph_row (MATRIX_ROW (f->desired_matrix, i));
> --8<---------------cut here---------------end--------------->8---

In the TTY case, the glyph memory (struct glyph) for desired window matrices is sub-allocated from the glyph memory that is allocated for the frame.  The glyph_row structures on the other hand are different between windows and frame.  Clear_glyph_row doesn't affect the glyph memory, only the glyh_rows.

> 
> 
> 4. And one more question: why don't Emacs is use the window feature of
> ncurses despite depending on it?  Is it inefficient or lack of any
> feature?
> 

The reasons I can think of are

- ncurses is considerably younger than Emacs
- Its predecessor 'curses' (from BSD, IIRC) wasn't available everywhere (think VMS, MS-DOS, maybe others), when that part of the code was written.  I would guess that might have been around 1985.  What would that be - Emacs 16 :-).

Maybe there are also other technical reasons that make a rewrite with ncurses impossible, I don'T know.

(And there's of course always the consideration of why rewrite something that works.)