Fix appending characters at end of line in double width lines

Since double-width/double-height lines have room for just half the
characters, take that into account.

Can be tested with:

perl -E '$r=join "", map{$_%10}1..80; say "\e<\e[?40h\e[?3l\e[?7h$r";
         $s="L"."X"x38 ."RL"; say "\e#6$s"'

The above tests that appending characters to double-width lines in
DECAWM (Auto Wrap Mode) wraps at the correct last column. There should
appear a second line (single-width) with a single "L".

perl -E '$r=join "", map{$_%10}1..80; say "\e<\e[?40h\e[?3l\e[?7l$r";
         $s="L"."X"x42 ." TEST FAILED OUTOFBOUNDS R"; say "\e#6$s"'

The above tests that appending characters to double-width lines in
non-DECAWM (Auto Wrap Mode) doesn't write past the last column (visible
on repaints - switch to another window and back). The last column (below
columns 79 and 80 of prev line) should have an "R".

src/Screen.cpp

@@ -716,6 +716,17 @@ QVector<LineProperty> Screen::getLineProperties(int startLine , int endLine) con
     return result;
 }
 
+int Screen::getScreenLineColumns(const int line) const
+{
+    const int doubleWidthLine = _lineProperties[line] & LINE_DOUBLEWIDTH;
+
+    if (doubleWidthLine) {
+        return _columns / 2;
+    }
+
+    return _columns;
+}
+
 void Screen::reset()
 {
     // Clear screen, but preserve the current line
@@ -891,12 +902,12 @@ void Screen::displayCharacter(uint c)
         return;
     }
 
-    if (_cuX + w > _columns) {
+    if (_cuX + w > getScreenLineColumns(_cuY)) {
         if (getMode(MODE_Wrap)) {
             _lineProperties[_cuY] = static_cast<LineProperty>(_lineProperties[_cuY] | LINE_WRAPPED);
             nextLine();
         } else {
-            _cuX = qMax(_columns - w, 0);
+            _cuX = qMax(getScreenLineColumns(_cuY) - w, 0);
         }
     }
 

src/Screen.h

@@ -666,6 +666,10 @@ private:
 
     int getLineLength(const int line) const;
 
+    // returns the width in columns of the specified screen line,
+    // taking DECDWL/DECDHL (double width/height modes) into account.
+    int getScreenLineColumns(const int line) const;
+
     // screen image ----------------
     int _lines;
     int _columns;