Keep scrollback under a full-screen scroll region

tests/test_emulator.py

@@ -141,6 +141,11 @@ def run_case(char_by_char):
     assert len(screen.history) == 1, (tag, len(screen.history))
     assert screen.history[0][0].data == "a", tag
 
+    # an explicit full-screen scroll region still feeds scrollback
+    proc, screen, stream = make(10, 3)
+    feed(stream, "\x1b[1;3ra\r\nb\r\nc\r\nd", char_by_char)
+    assert len(screen.history) == 1, (tag, len(screen.history))
+
     # ED 3 clears history and fires callback
     cleared = []
     proc = FakeProcess()

vterm/emulator.py

@@ -442,7 +442,9 @@ class VtScreen(pyte.Screen):
 
     def index(self):
         """Line feed; at the bottom of the primary screen the top row goes to scrollback."""
-        if not self.alternate_buffer_mode and self.cursor.y == self.lines - 1 and self.margins is None:
+        # a region spanning the whole screen (CSI 1;<lines> r) is no region
+        full = self.margins is None or self.margins == (0, self.lines - 1)
+        if not self.alternate_buffer_mode and self.cursor.y == self.lines - 1 and full:
             self.push_lines_into_history(1)
         super().index()