tui: Disable BCE almost always. #7624

133ae5eeef implemented BCE (background color erase). But we can't
trust terminfo, so it is safer disable BCE if we are not certain.

Per https://github.com/kovidgoyal/kitty/issues/160#issuecomment-346470545
terminal support for BCE seems to be (1) optional and (2) inconsistent.

ref #4210 #4421 #7035 #7337 #7381 #7425 #7618

src/nvim/tui/tui.c

@@ -1234,11 +1234,9 @@ static int unibi_find_ext_str(unibi_term *ut, const char *name)
   return -1;
 }
 
-/// Several entries in terminfo are known to be deficient or outright wrong,
+/// Patches the terminfo records after loading from system or built-in db.
-/// unfortunately; and several terminal emulators falsely announce incorrect
+/// Several entries in terminfo are known to be deficient or outright wrong;
-/// terminal types.  So patch the terminfo records after loading from an
+/// and several terminal emulators falsely announce incorrect terminal types.
-/// external or a built-in database.  In an ideal world, the real terminfo data
-/// would be correct and complete, and this function would be almost empty.
 static void patch_terminfo_bugs(TUIData *data, const char *term,
                                 const char *colorterm, long vte_version,
                                 bool konsole, bool iterm_env)
@@ -1304,6 +1302,11 @@ static void patch_terminfo_bugs(TUIData *data, const char *term,
     }
   }
 
+  if (!true_xterm) {
+    // Cannot trust terminfo; safer to disable BCE. #7624
+    unibi_set_bool(ut, unibi_back_color_erase, false);
+  }
+
   if (xterm) {
     // Termit, LXTerminal, GTKTerm2, GNOME Terminal, MATE Terminal, roxterm,
     // and EvilVTE falsely claim to be xterm and do not support important xterm
@@ -1397,11 +1400,9 @@ static void patch_terminfo_bugs(TUIData *data, const char *term,
 #define XTERM_SETAB_16 \
   "\x1b[%?%p1%{8}%<%t4%p1%d%e%p1%{16}%<%t10%p1%{8}%-%d%e39%;m"
 
-  // Terminals where there is actually 256-colour SGR support despite what
+  // Terminals with 256-colour SGR support despite what terminfo says.
-  // the terminfo record may say.
   if (unibi_get_num(ut, unibi_max_colors) < 256) {
-    // See http://fedoraproject.org/wiki/Features/256_Color_Terminals for
+    // See http://fedoraproject.org/wiki/Features/256_Color_Terminals
-    // more on this.
     if (true_xterm || iterm || iterm_pretending_xterm) {
       unibi_set_num(ut, unibi_max_colors, 256);
       unibi_set_str(ut, unibi_set_a_foreground, XTERM_SETAF_256_COLON);
@@ -1417,8 +1418,7 @@ static void patch_terminfo_bugs(TUIData *data, const char *term,
       unibi_set_str(ut, unibi_set_a_background, XTERM_SETAB_256);
     }
   }
-  // Terminals where there is actually 16-colour SGR support despite what
+  // Terminals with 16-colour SGR support despite what terminfo says.
-  // the terminfo record may say.
   if (unibi_get_num(ut, unibi_max_colors) < 16) {
     if (colorterm) {
       unibi_set_num(ut, unibi_max_colors, 16);
@@ -1427,9 +1427,8 @@ static void patch_terminfo_bugs(TUIData *data, const char *term,
     }
   }
 
-  // Some terminals can not currently be trusted to report if they support
+  // Some terminals cannot be trusted to report DECSCUSR support. So we keep
-  // DECSCUSR or not. So we need to have a blacklist for when we should not
+  // blacklist for when we should not trust the reported features.
-  // trust the reported features.
   if (!((vte_version != 0 && vte_version < 3900) || konsole)) {
     // Dickey ncurses terminfo has included the Ss and Se capabilities,
     // pioneered by tmux, since 2011-07-14. So adding them to terminal types,