fix for vim glitch

tests/test_render.py

@@ -202,6 +202,24 @@ command.trim_history(None, terminal)
 assert "row 0" in terminal.view.text and "row 7" in terminal.view.text, \
     terminal.view.text
 
+# nor may its blank rows be trimmed: an app that blanks and refills its last
+# row (vim's showcmd corner) would resize the view under a pinned viewport
+terminal, stream = make_terminal(rows=4)
+stream.feed("\x1b[?1049h")
+terminal.view = FakeView("top\nrow\n\n")
+command = make_command(terminal.view)
+before = terminal.view.text
+command.trim_trailing_spaces(None, terminal)
+assert terminal.view.text == before, \
+    "alt screen blank tail was trimmed: {!r}".format(terminal.view.text)
+
+# on the primary screen the blank tail below the cursor still goes
+terminal, stream = make_terminal(rows=4)
+terminal.view = FakeView("top\nrow\n\n")
+command = make_command(terminal.view)
+command.trim_trailing_spaces(None, terminal)
+assert terminal.view.text == "top\nrow", terminal.view.text
+
 print("render tests passed (alt screen keeps its view, cap honoured)")
 
 

vterm/keys.py

@@ -105,8 +105,7 @@ def _modifier_code(ctrl, alt, shift):
     return 1 + (1 if shift else 0) + (2 if alt else 0) + (4 if ctrl else 0)
 
 
-def get_key_code(key, ctrl=False, alt=False, shift=False, application_mode=False, new_line_mode=False,
-                 modify_other_keys=0):
+def get_key_code(key, ctrl=False, alt=False, shift=False, application_mode=False, new_line_mode=False, modify_other_keys=0):
     """Escape sequence for a key name and its modifiers, honouring the app's active modes"""
     key_lo = key.lower() if len(key) > 1 else key
 

vterm/render.py

@@ -410,9 +410,16 @@ class VtermRenderCommand(sublime_plugin.TextCommand):
 
         Without this the view keeps a full screen of blank rows below a short
         prompt, and the scrollbar reflects them.
+
+        Both are skipped on the alternate screen, where the app owns every row:
+        one that blanks and refills - vim's showcmd corner, on every keypress -
+        would resize the view under a viewport pinned to its bottom, and the
+        whole screen shifts by the part of a row the viewport leaves over.
         """
         view = self.view
         screen = terminal.screen
+        if screen.alternate_buffer_mode:
+            return
         cursor = screen.cursor
         cursor_row = terminal.offset + cursor.y
         lastrow = view.rowcol(view.size())[0]