patch 9.0.1543: display errors when making topline shorter

Problem:    Display errors when making topline shorter and 'smoothscroll' is
            set.
Solution:   Reset w_skipcol when the topline becomes shorter than its current
            value. (Luuk van Baal, closes #12367)

src/testdir/dumps/Test_display_long_line_1.dump

@@ -9,6 +9,6 @@
 @35
 @35
 @35
-@26>a@8
-@10| @24
-@18|1|,|4|8|2| @7|T|o|p| 
+@35
+>a@14| @19
+@18|1|,|7|3|6| @7|T|o|p| 

src/testdir/dumps/Test_display_long_line_2.dump

@@ -9,6 +9,6 @@
 @35
 @35
 @35
-@25>a| @8
+@34>a
 |b@4| |b@4| |b@4| |b@4| |b@4| |b@1|@+0#4040ff13&@2
-| +0#0000000&@17|1|,|4|8|1| @7|T|o|p| 
+| +0#0000000&@17|1|,|7|3|5| @7|T|o|p| 

src/testdir/dumps/Test_display_long_line_3.dump

@@ -0,0 +1,14 @@
+|<+0#4040ff13#ffffff0@2>a+0#0000000&| @30
+|b@4| |b@4| |b@4| |b@4| |b@4| |b@4
+| |b@4| |c@4| |c@4| |c@4| |c@4| |c@3
+@1| |c@4| |c@4| |d@4| |d@4| |d@4| |d@2
+@2| |d@4| |d@4| |d@4| @14
+|~+0#4040ff13&| @33
+|~| @33
+|~| @33
+|~| @33
+|~| @33
+|~| @33
+|~| @33
+|~| @33
+| +0#0000000&@17|1|,|3|1|9| @7|A|l@1| 

src/testdir/dumps/Test_display_long_line_4.dump

@@ -0,0 +1,14 @@
+|a+0&#ffffff0@34
+@35
+@35
+@35
+@35
+@35
+@35
+@35
+@35
+@2>a| @31
+|b@4| |b@4| |b@4| |b@4| |b@4| |b@4
+| |b@4| |c@4| |c@4| |c@4| |c@4| |c@3
+@1| |c@4| |c@4| |d@4| |d@4| |d@4| |@+0#4040ff13&@2
+| +0#0000000&@17|1|,|3|1|8| @7|T|o|p| 

src/testdir/dumps/Test_smooth_long_15.dump

@@ -2,5 +2,5 @@
 |h| |l|o|t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o
 |t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o
 |f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o|f| |t|e|x|t| |e|n|d| @11
-|f|o|u>r| @35
-@22|4|,|4| @10|B|o|t| 
+>f|o|u|r| @35
+@22|4|,|1| @10|B|o|t| 

src/testdir/dumps/Test_smooth_long_16.dump

@@ -0,0 +1,6 @@
+|<+0#4040ff13#ffffff0@2|t+0#0000000&|h| |l|o|t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o|f| |t|e|x|t| |w|i|t
+|h| |l|o|t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o
+|t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o|f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o
+|f| |t|e|x|t| |w|i|t|h| |l|o|t|s| |o|f| |t|e|x|t| |e|n|d| @11
+|f|o|u>r| @35
+@22|4|,|4| @10|B|o|t| 

src/testdir/test_display.vim

@@ -471,9 +471,9 @@ func Test_display_long_lastline()
   CheckScreendump
 
   let lines =<< trim END
-    set display=lastline
+    set display=lastline smoothscroll scrolloff=0
     call setline(1, [
-      \'aaaaa'->repeat(100),
+      \'aaaaa'->repeat(150),
       \'bbbbb '->repeat(7) .. 'ccccc '->repeat(7) .. 'ddddd '->repeat(7)
     \])
   END
@@ -481,11 +481,23 @@ func Test_display_long_lastline()
   call writefile(lines, 'XdispLongline', 'D')
   let buf = RunVimInTerminal('-S XdispLongline', #{rows: 14, cols: 35})
 
-  call term_sendkeys(buf, "482|")
+  call term_sendkeys(buf, "736|")
   call VerifyScreenDump(buf, 'Test_display_long_line_1', {})
+
+  " The correct part of the last line is moved into view.
   call term_sendkeys(buf, "D")
   call VerifyScreenDump(buf, 'Test_display_long_line_2', {})
 
+  " "w_skipcol" does not change because the topline is still long enough
+  " to maintain the current skipcol.
+  call term_sendkeys(buf, "g04l11gkD")
+  call VerifyScreenDump(buf, 'Test_display_long_line_3', {})
+
+  " "w_skipcol" is reset to bring the entire topline into view because
+  " the line length is now smaller than the current skipcol + marker.
+  call term_sendkeys(buf, "x")
+  call VerifyScreenDump(buf, 'Test_display_long_line_4', {})
+
   call StopVimInTerminal(buf)
 endfunc
 

src/testdir/test_scroll_opt.vim

@@ -335,9 +335,12 @@ func Test_smoothscroll_wrap_long_line()
   " than one window. Note that the cursor is at the bottom this time because
   " Vim prefers to do so if we are scrolling a few lines only.
   call term_sendkeys(buf, ":call setline(1, ['one', 'two', 'Line' .. (' with lots of text'->repeat(10)) .. ' end', 'four'])\<CR>")
+  " Currently visible lines were replaced, test that the lines and cursor
+  " are correctly displayed.
+  call VerifyScreenDump(buf, 'Test_smooth_long_14', {})
   call term_sendkeys(buf, "3Gzt")
   call term_sendkeys(buf, "j")
-  call VerifyScreenDump(buf, 'Test_smooth_long_14', {})
+  call VerifyScreenDump(buf, 'Test_smooth_long_15', {})
 
   " Repeat the step but this time start it when the line is smooth-scrolled by
   " one line. This tests that the offset calculation is still correct and
@@ -345,7 +348,7 @@ func Test_smoothscroll_wrap_long_line()
   " screen.
   call term_sendkeys(buf, "3Gzt")
   call term_sendkeys(buf, "\<C-E>j")
-  call VerifyScreenDump(buf, 'Test_smooth_long_15', {})
+  call VerifyScreenDump(buf, 'Test_smooth_long_16', {})
 
   call StopVimInTerminal(buf)
 endfunc