TeraTerm Bug, scroll buffer cannot be cleared

TERATERM BUG REPORT

Submit at: https://github.com/TeraTermProject/teraterm/issues/new

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Title: Alternate screen buffer (DECSET 1049) and CSI 3 J not working over serial connection

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Environment

• Tera Term version: 5.5.1
• Windows version: [your Windows version]
• Connection type: Serial port (USB CDC, 115200 baud)

Problem Description

The alternate screen buffer escape sequences (ESC[?1049h / ESC[?1049l) do not work when connected via serial port. Content written while in "alternate screen" mode appears in the main scroll buffer instead of a separate buffer, and exiting alternate screen mode does not restore the previous screen contents.

Additionally, the CSI 3 J (Erase Saved Lines / clear scroll buffer) sequence has no effect, even with ClearScrollBufferFromRemote=on set in TERATERM.INI.

TERATERM.INI Settings

AlternateScreenBuffer=on
ClearScrollBufferFromRemote=on

Steps to Reproduce

1. Connect to a device via serial port (e.g., COM3 at 115200 baud)
2. Device sends: ESC[?1049h (enter alternate screen buffer)
3. Device sends: ESC[2J ESC[H (clear screen, home cursor)
4. Device displays a full-screen application (e.g., text editor)
5. User exits application
6. Device sends: ESC[?1049l (exit alternate screen buffer)

Expected Behavior

• Step 2-4: Application displays on alternate screen
• Step 6: Previous screen content (before step 2) is restored, application content disappears

Actual Behavior

• All content appears in the main scroll buffer
• Exiting alternate screen mode does not restore previous content
• Scrolling up shows remnants of the full-screen application mixed with previous output
• ESC[3J does not clear the scroll buffer

Additional Context

This is for an embedded Forth system (Raspberry Pi Pico) that uses VT100 escape sequences for full-screen editors. The same escape sequences work correctly in other terminal emulators (e.g., xterm, Windows Terminal).

The application sends:

• On entry: \x1b[?1049h then \x1b[2J\x1b[H
• On exit: \x1b[?1049l

Workaround Attempted

Tried using ESC[3J (clear scroll buffer) before and after the application, with ClearScrollBufferFromRemote=on - no effect.

[hkanou]

Thank you for reporting the bug.
The bug report is clear and very helpful.

1. The alternate screen buffer escape sequences (ESC[?1049h / ESC[?1049l)
   Could you please set the TERM environment variable to xterm and check whether the behavior is correct?
   Example: in bash run export TERM=xterm

2. The CSI 3 J (Erase Saved Lines / clear scroll buffer) sequence
   Could you run echo -e '\e[3J' and check whether it produces the expected behavior?
   Also, please verify that the storage location for TERATERM.INI is the one you intend under [Setup] > [Setup directory] > Term Term Configuration File.