from https://groups.google.com/a/chromium.org/d/topic/chromium-hterm/OVTfVnQeoW8/discussion
> The scrollback is not updated on CSI 2J. This sequence is usually sent by readline when you type ^L, and people often use ^L as a way to clear the screen while keeping scrollback. Hterm shows the same behavior as xterm and the Linux virtual console (on the virtual console ^L sends a parameterless CSI J, which has the same scrollback behavior), but it seems less useful than the alternatives. Konsole (and derivatives like QTerm) and VTE-based emulators like Gnome Terminal push the entire current screen into the scrollback buffer, which leads to a lot of blank lines, but still leaves everything findable. The ideal behavior seems to be what screen and tmux do: push only up to the last line with content. I have a workaround for this when using zsh, which uses its own ZLE instead of the standard Readline, but Readline doesn’t seem to have similar customization options.

having hterm behave like screen/tmux sounds reasonable