patch 8.2.1963: crash when using a popup window with "latin1" encoding

Problem:    Crash when using a popup window with "latin1" encoding.
Solution:   Don't use ScreenLinesUC when enc_utf8 is false. (closes #7241)

src/screen.c

@@ -464,7 +464,8 @@ screen_line(
     // First char of a popup window may go on top of the right half of a
     // double-wide character. Clear the left half to avoid it getting the popup
     // window background color.
-    if (coloff > 0 && ScreenLines[off_to] == 0
+    if (coloff > 0 && enc_utf8
+		   && ScreenLines[off_to] == 0
 		   && ScreenLinesUC[off_to - 1] != 0
 		   && (*mb_char2cells)(ScreenLinesUC[off_to - 1]) > 1)
     {

src/terminal.c

@@ -1830,6 +1830,10 @@ update_snapshot(term_T *term)
 			width = cell.width;
 
 			cell2cellattr(&cell, &p[pos.col]);
+			if (width == 2)
+			    // second cell of double-width character has the
+			    // same attributes.
+			    p[pos.col + 1] = p[pos.col];
 
 			// Each character can be up to 6 bytes.
 			if (ga_grow(&ga, VTERM_MAX_CHARS_PER_CELL * 6) == OK)
@@ -3639,6 +3643,7 @@ term_line2screenline(
 	    }
 #endif
 	    else
+		// This will only store the lower byte of "c".
 		ScreenLines[off] = c;
 	}
 	ScreenAttrs[off] = cell2attr(term, wp, cell.attrs, cell.fg, cell.bg);
@@ -3647,13 +3652,20 @@ term_line2screenline(
 	++off;
 	if (cell.width == 2)
 	{
-	    if (enc_utf8)
-		ScreenLinesUC[off] = NUL;
-
 	    // don't set the second byte to NUL for a DBCS encoding, it
 	    // has been set above
-	    if (enc_utf8 || !has_mbyte)
+	    if (enc_utf8)
+	    {
+		ScreenLinesUC[off] = NUL;
 		ScreenLines[off] = NUL;
+	    }
+	    else if (!has_mbyte)
+	    {
+		// Can't show a double-width character with a single-byte
+		// 'encoding', just use a space.
+		ScreenLines[off] = ' ';
+		ScreenAttrs[off] = ScreenAttrs[off - 1];
+	    }
 
 	    ++pos->col;
 	    ++off;

src/testdir/test_popupwin.vim

@@ -3685,6 +3685,28 @@ func Test_popupwin_filter_close_three_errors()
   call delete('XtestPopupThreeErrors')
 endfunc
 
+func Test_popupwin_latin1_encoding()
+  CheckScreendump
+  CheckUnix
+
+  " When 'encoding' is a single-byte encoding a terminal window will mess up
+  " the display.  Check that showing a popup on top of that doesn't crash.
+  let lines =<< trim END
+      set encoding=latin1
+      terminal cat Xmultibyte
+      call popup_create(['one', 'two', 'three', 'four'], #{line: 1, col: 10})
+  END
+  call writefile(lines, 'XtestPopupLatin')
+  call writefile([repeat("\u3042 ", 120)], 'Xmultibyte')
+
+  let buf = RunVimInTerminal('-S XtestPopupLatin', #{rows: 10})
+
+  call term_sendkeys(buf, ":q\<CR>")
+  call StopVimInTerminal(buf)
+  call delete('XtestPopupLatin')
+  call delete('Xmultibyte')
+endfunc
+
 func Test_popupwin_atcursor_far_right()
   new
 

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1963,
 /**/
     1962,
 /**/