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, 5 insertions, 6 deletions

diff --git a/test/functional/ui/cmdline_spec.lua b/test/functional/ui/cmdline_spec.lua
index e86414fe5f..0ebb62f78f 100644
--- a/test/functional/ui/cmdline_spec.lua
+++ b/test/functional/ui/cmdline_spec.lua
@@ -253,7 +253,7 @@ local function test_cmdline(linegrid)
     ]], cmdline=expectation}
 
     -- erase information, so we check if it is retransmitted
-    command("redraw!")
+    command("mode")
     screen:expect{grid=[[
       ^                         |
       {1:~                        }|
@@ -320,7 +320,7 @@ local function test_cmdline(linegrid)
       {{'  line1'}},
     }}
 
-    command("redraw!")
+    command("mode")
     screen:expect{grid=[[
       ^                         |
       {1:~                        }|
@@ -411,7 +411,7 @@ local function test_cmdline(linegrid)
       pos = 4,
     }}}
 
-    command("redraw!")
+    command("mode")
     screen:expect{grid=[[
                                |
       {2:[No Name]                }|
@@ -435,9 +435,9 @@ local function test_cmdline(linegrid)
 
     feed("<c-c>")
     screen:expect{grid=[[
-                               |
+      ^                         |
       {2:[No Name]                }|
-      {1::}make^                    |
+      {1::}make                    |
       {3:[Command Line]           }|
                                |
     ]], cmdline={{
@@ -446,7 +446,6 @@ local function test_cmdline(linegrid)
       pos = 4,
     }}}
 
-    screen.cmdline = {}
     command("redraw!")
     screen:expect{grid=[[
       ^                         |