ui: disable clearing almost everywhere

Avoid clearing the screen in most situations. NOT_VALID should be
equivalent to CLEAR unless some external force messed up the terminal,
for these situations <c-l> and :mode will still clear the screen.

Also eliminate some obsolete code in screen.c, that dealt with that in
vim drawing window 1 can mess up window 2, but this never happens in
nvim.

But what about slow terminals? There is two common meanings in which
a terminal is said to be "slow":

Most commonly (and in the sense of vim:s nottyfast) it means low
bandwidth for sending bytes from nvim to the terminal. If the screen is
very similar before and after the update_screen(CLEAR) this change
should reduce bandwidth. If the screen is quite different, but there is
no new regions of contiguous whitespace, clearing doesn't reduce
bandwidth significantly. If the new screen contains a lot of whitespace,
it will depend of if vsplits are used or not: as long as there is no
vsplits, ce is used to cheaply clear the rest of the line, so
full-screen clear is not needed to reduce bandwith. However a left
vsplit currently needs to be padded with whitespace all the way to the
separator. It is possible ec (clear N chars) can be used to reduce
bandwidth here if this is a problem. (All of this assumes that one
doesn't set Normal guibg=... on a non-BCE terminal, if you do you are
doomed regardless of this change).

Slow can also mean that drawing pixels on the screen is slow. E-ink
screens is a recent example. Avoiding clearing and redrawing the
unchanged part of the screen will always improve performance in these
cases.

1 files changed, 12 insertions, 12 deletions

diff --git a/src/nvim/window.c b/src/nvim/window.c
index e541e4bb68..56caf7a941 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -1265,7 +1265,8 @@ static void win_exchange(long Prenum)
   (void)win_comp_pos();                 /* recompute window positions */
 
   win_enter(wp, true);
-  redraw_later(CLEAR);
+  redraw_later(NOT_VALID);
+  redraw_win_later(wp, NOT_VALID);
 }
 
 /*
@@ -1340,7 +1341,7 @@ static void win_rotate(int upwards, int count)
     (void)win_comp_pos();
   }
 
-  redraw_later(CLEAR);
+  redraw_all_later(NOT_VALID);
 }
 
 /*
@@ -1477,10 +1478,10 @@ static void win_equal_rec(
         || topfr->fr_width != width || topfr->fr_win->w_wincol != col
         ) {
       topfr->fr_win->w_winrow = row;
-      frame_new_height(topfr, height, FALSE, FALSE);
+      frame_new_height(topfr, height, false, false);
       topfr->fr_win->w_wincol = col;
-      frame_new_width(topfr, width, FALSE, FALSE);
-      redraw_all_later(CLEAR);
+      frame_new_width(topfr, width, false, false);
+      redraw_all_later(NOT_VALID);
     }
   } else if (topfr->fr_layout == FR_ROW) {
     topfr->fr_width = width;
@@ -3105,7 +3106,7 @@ int win_new_tabpage(int after, char_u *filename)
     newtp->tp_topframe = topframe;
     last_status(FALSE);
 
-    redraw_all_later(CLEAR);
+    redraw_all_later(NOT_VALID);
 
     apply_autocmds(EVENT_WINNEW, NULL, NULL, false, curbuf);
     apply_autocmds(EVENT_WINENTER, NULL, NULL, false, curbuf);
@@ -3310,10 +3311,9 @@ static void enter_tabpage(tabpage_T *tp, buf_T *old_curbuf, int trigger_enter_au
                 trigger_enter_autocmds, trigger_leave_autocmds);
   prevwin = next_prevwin;
 
-  last_status(FALSE);           /* status line may appear or disappear */
-  (void)win_comp_pos();         /* recompute w_winrow for all windows */
-  must_redraw = CLEAR;          /* need to redraw everything */
-  diff_need_scrollbind = TRUE;
+  last_status(false);  // status line may appear or disappear
+  (void)win_comp_pos();  // recompute w_winrow for all windows
+  diff_need_scrollbind = true;
 
   /* The tabpage line may have appeared or disappeared, may need to resize
    * the frames for that.  When the Vim window was resized need to update
@@ -3335,7 +3335,7 @@ static void enter_tabpage(tabpage_T *tp, buf_T *old_curbuf, int trigger_enter_au
       apply_autocmds(EVENT_BUFENTER, NULL, NULL, FALSE, curbuf);
   }
 
-  redraw_all_later(CLEAR);
+  redraw_all_later(NOT_VALID);
 }
 
 /*
@@ -5443,7 +5443,7 @@ restore_snapshot (
     win_comp_pos();
     if (wp != NULL && close_curwin)
       win_goto(wp);
-    redraw_all_later(CLEAR);
+    redraw_all_later(NOT_VALID);
   }
   clear_snapshot(curtab, idx);
 }