Total Terminal opens new tab separately in macOS High Sierra

According to macOS, windows should not be behind the Dock. When the bounds of a window are updated, macOS moves any windows from behind the Dock above the Dock.

The window bounds update when you add a new tab, so macOS looks at those bounds and moves them such that they don't overlap with the Dock section of the screen. There's no way to change this currently.

As for why this happens in High Sierra and not in Sierra, I presume it's because Terminal now uses the unified tabs functionality provided in AppKit.

Terminal has always had tabs, but macOS recently introduced tabs as part of the OS, for any app to use. I believe Terminal now uses this functionality for its tabs, which has caused this. You can test this by opening Terminal with multiple tabs and pressing ⇧⌘\ for Show All Tabs — this is functionality provided system-wide by the OS, not by Terminal, for managing tabs including the tab bar.

After some googling, I found this script on pastebin from an anonymous user. I haven't had a chance to try it myself, but it looks like it has potential with some modifications.

# Separate Command Histories for Restored Terminal Sessions

# Terminal assigns each terminal session a unique identifier and
# communicates it via the TERM_SESSION_ID environment variable so that
# programs running in a terminal can save/restore application-specific
# state when quitting and restarting Terminal with Resume enabled.

# The following script saves and restores the bash command history
# independently for each restored terminal session. It also merges
# commands into the global history for new sessions. Because of this
# it is recommended that you set HISTSIZE and HISTFILESIZE to larger
# values (their default value is 500). Old sessions are periodically
# deleted.

if [ -z "$BASH_SESSION" ] && [ -n "$TERM_SESSION_ID" ]; then

    # Only perform this setup once per shell session (which shouldn't
    # happen unless the user's ~/.bash_profile executes /etc/profile,
    # which is normally redundant).
    BASH_SESSION=1

    # Set up the session directory/file.
    if [ -z "$BASH_SESSION_DIR" ]; then
        BASH_SESSION_DIR="$HOME/.bash_sessions"
        BASH_SESSION_FILE="$BASH_SESSION_DIR/$TERM_SESSION_ID.session"
    elif [ -z "$BASH_SESSION_FILE" ]; then
        BASH_SESSION_FILE="$BASH_SESSION_DIR/$TERM_SESSION_ID.session"
    fi
    mkdir -p "$BASH_SESSION_DIR"

    # Arrange for session-specific shell command history. Users can
    # disable this via the existence of the following file.
    [ -f ~/.bash_session_no_history ] && BASH_SESSION_HISTORY=0 || BASH_SESSION_HISTORY=1
    if [ $BASH_SESSION_HISTORY -eq 1 ]; then
        BASH_SHARED_HISTFILE="$HISTFILE"
        BASH_SESSION_HISTFILE="$BASH_SESSION_DIR/$TERM_SESSION_ID.history"
        BASH_SESSION_HISTFILE_NEW="$BASH_SESSION_DIR/$TERM_SESSION_ID.historynew"
        # If the session history doesn't exist, copy the shared history
        if [ -f "$BASH_SHARED_HISTFILE" ] && [ ! -f "$BASH_SESSION_HISTFILE" ]; then
            cp "$BASH_SHARED_HISTFILE" "$BASH_SESSION_HISTFILE"
        else
            # Ensure the file exists and doesn't get expired.
            touch "$BASH_SESSION_HISTFILE"
        fi
        history -r "$BASH_SESSION_HISTFILE"
        : >| "$BASH_SESSION_HISTFILE_NEW"
        HISTFILE="$BASH_SESSION_HISTFILE_NEW"
    fi

    if [ "$SHLVL" -eq 1 ]; then
        # Restore previous session state.
        if [ -f "$BASH_SESSION_FILE" ]; then
            . "$BASH_SESSION_FILE"
            rm "$BASH_SESSION_FILE"
        fi

        # Save the current state.
        bash_save_session_state() {
            if [ "$SHLVL" -eq 1 ] && [ -n "$BASH_SESSION_FILE" ]; then
                echo -n Saving session...
                echo echo Restored session: $(date) > "$BASH_SESSION_FILE"

                # Users can add custom state by defining the following
                # function. e.g., to save an environment variable:
                #   bash_session_save_state() { echo MY_VAR="'$MY_VAR'" >> "$BASH_SESSION_FILE"; }
                declare -F bash_session_save_state >/dev/null && bash_session_save_state

                # Save new history commands.
                if [ $BASH_SESSION_HISTORY -eq 1 ]; then
                    history -a
                    cat "$BASH_SESSION_HISTFILE_NEW" >> "$BASH_SHARED_HISTFILE"
                    cat "$BASH_SESSION_HISTFILE_NEW" >> "$BASH_SESSION_HISTFILE"
                    # Empty this session's history file to keep track of
                    # which commands have already been copied.
                    : >| "$BASH_SESSION_HISTFILE_NEW"
                    # Read/write the files via the history command so they
                    # are truncated as appropriate.
                    history -r "$BASH_SHARED_HISTFILE"
                    history -w "$BASH_SHARED_HISTFILE"
                    history -r "$BASH_SESSION_HISTFILE"
                    history -w "$BASH_SESSION_HISTFILE"
                fi

                echo completed.
            fi
        }

        # Delete old session files. Do not do this more frequently
        # than once a day.
        BASH_SESSION_TIMESTAMP_FILE="$BASH_SESSION_DIR/_expiration_check_timestamp"
        bash_delete_expired_session_state() {
            if ([ ! -f "$BASH_SESSION_TIMESTAMP_FILE" ] || [ -z $(find "$BASH_SESSION_TIMESTAMP_FILE" -mtime -1d) ]); then
                local bash_session_lock_file="$BASH_SESSION_DIR/_expiration_lockfile"
                if shlock -f "$bash_session_lock_file" -p $$; then
                    echo -n Deleting expired sessions...
                    local delete_count=$(find "$BASH_SESSION_DIR" -type f -mtime +2w -print -delete | wc -l)
                    [ "$delete_count" -gt 0 ] && echo $delete_count completed. || echo none found.
                    touch "$BASH_SESSION_TIMESTAMP_FILE"
                    rm "$bash_session_lock_file"
                fi
            fi
        }

        # Update saved session state when exiting.
        bash_update_session_state() {
            bash_save_session_state && bash_delete_expired_session_state
        }
        trap bash_update_session_state EXIT
    fi
fi