ExecProcess with AltScreen shows output of previous run of ExecProcess for a few milliseconds

[flaw]

I'm doing a simple thing that allows one to select a git commit from a list, and then launch a diff tool to show it.
For context, in the past this has been a shell one-liner: git log --since 1day --format=format:%H | xargs -I % sh -c 'git show % | delta --paging always', there we could move to next commit by closing delta, but there was no way of browsing back, thus bubbletea.

This is all cobbled together from the tutorial and the exec/altscreen examples, but changing the openEditor function to:

func openDiff(sha string) tea.Cmd {
	c := exec.Command("git", "show", sha) //nolint:gosec
	diffCmd := tea.ExecProcess(c, func(err error) tea.Msg {
		return diffFinishedMsg{err}
	})
	return diffCmd
}

I have a hard-coded list of commits SHAs in the initial model. When I pick a commit from the list and pass it into a tea.ExecProcess, for a few milliseconds I see the diff of the previous commit (or my terminal as it was before starting the bbt process if this is the first commit being picked) before the content is updated with the correct content. I've tried playing around with returning a tea.Sequence(tea.ClearScreen, diffCmd) but that had no observable impact.

Any ideas how I could get it to not show the wrong output when running ExecProcess? I would be entirely happy even with a blank screen while it's waiting for the output of the process being run, but showing the previous diff makes for a jarring experience.

[adilburaksen]

This is a known quirk of how ExecProcess hands off to the external process. When bubbletea suspends its renderer and passes control to the child process, the terminal still has whatever was last drawn in it — the new process hasn't written anything yet, so for a few milliseconds you see the stale content.

tea.Sequence(tea.ClearScreen, diffCmd) doesn't reliably fix this because the clear is queued through bubbletea's renderer and may not be flushed to the terminal before the handoff happens.

The straightforward workaround is to clear the screen inside the command itself:

func openDiff(sha string) tea.Cmd {
    c := exec.Command("bash", "-c", "printf '\\033[2J\\033[H' && git show "+sha+" | delta --paging always")
    return tea.ExecProcess(c, func(err error) tea.Msg {
        return diffFinishedMsg{err}
    })
}

The printf '\033[2J\033[H' clears the screen and moves the cursor to the top-left before delta starts, so there's nothing stale to flash.

Alternatively, keep it simpler with clear:

c := exec.Command("bash", "-c", "clear; git show "+sha+" | delta --paging always")

If you're already running inside AltScreen, the root cause is the same — bubbletea doesn't clear before handing off to the subprocess, so the old frame is still visible until the child process draws over it.