Display Misalignment Issue with Corfu in TTY-Child-Frames

[luluman]

Thank you for providing this excellent completion package and adding support for tty-child-frames. It's fantastic to have a native popup solution in the terminal, enhancing efficiency and robustness.

I've tested this feature and found a display alignment bug in Corfu. Using the Emacs build from commit 196c761c66, the completion popup appears one character to the left of the intended position. I attempted to resolve the issue by setting (internal-border-width . 0) but was unable to fix it.

Steps to Reproduce:

1. Compile Emacs using commit 196c761c66.
2. Install the Corfu package and attempt to complete a simple word.

[minad]

Thank you for testing this with the new tty feature. Obviously this feature is not yet fully supported by Corfu given that we are talking about a separate branch of the Emacs development repository. We will revisit this in the future when child frame tty support has been merged into the Emacs mainline. In the mean time, any help, testing and fixes are appreciated.

[galeo]

Do you think using decoration for the Corfu popup is a good idea or does it waste too much screen real estate? I quickly tried it and I am not that convinced.

Decoration doesn’t seem essential for the Corfu popup, and I’m satisfied with how it looks now.

I've tested on TERM=xterm-256color and TERM=xterm-direct (24bit) and there the faces obviously work well. The default Corfu colors are taken from an old version of the Modus themes. I think supporting terminals with fewer colors is difficult.

The current color setup is also acceptable for me. Since there’s no border, the corfu-popupinfo and the candidate popup share the same background, and together with the scrollbar and corfu-current background, the overall look can feel a bit cluttered. But it’s still usable, and I guess it just takes some getting used to.

For comparison, here’s a screenshot from Neovim.

Just adding this as a reference. Its completion popup has a border, which makes the visual structure a bit easier to parse. If Corfu could use a border instead of relying solely on background colors to define the boundaries, it might make things visually clearer — even if it costs a bit of space. I experimented with borders before and got a similar result, but I got busy and didn’t follow up on it.

[minad]

Okay, I think I would be fine with adding borders if we make it customizable, e.g., corfu-border-on-tty. In the screenshot above I would prefer if the main popup and the info popup share a common border, as is the case in Corfu on GUI.

[galeo]

A quick test. After adding a border to the popup and removing its background color, here’s the result.

Horizontally stacked:

Vertically stacked:

In the screenshot above I would prefer if the main popup and the info popup share a common border, as is the case in Corfu on GUI.

As shown in the screenshots above, there are gaps between the borders—especially when the two popups are vertically stacked. This has to do with the font used for the border-drawing characters. I don’t know how to make two popups share a common border currently. The popup border characters always include some padding around them, so the border lines don’t join cleanly and there will be visible gaps. I’m not sure whether it’s possible to work around this by manually drawing the borders using characters like ├ ┤ ┬ ┴, but that would get quite complicated.

[minad]

A quick test. After adding a border to the popup and removing its background color, here’s the result.

Happy to take a PR. I hope we don't have to add much more special casing to the already complicated frame position computations?

As shown in the screenshots above, there are gaps between the borders—especially when the two popups are vertically stacked.

Yes, but theese gaps cannot be avoided right? They are due to the borders drawn in separate lines?

The popup border characters always include some padding around them, so the border lines don’t join cleanly and there will be visible gaps. I’m not sure whether it’s possible to work around this by manually drawing the borders using characters like ├ ┤ ┬ ┴, but that would get quite complicated.

Right. We could ask Emacs to use joins if child frames overlap, but that's maybe asking too much, and too difficult to implement. Given that Neovim renders the popups also like this, we should be fine with separate frames.

[galeo]

Happy to take a PR. I hope we don't have to add much more special casing to the already complicated frame position computations?

I’ll try to prepare a pull request this weekend.

[amosbird]

The completion popup also misbehaves when displayed above the cursor. It leaves visible gap lines between the popup and the cursor. Additionally, when completion is aborted in such cases, the child frame sometimes remains visible, resembling a lingering shadow overlay effect.

emacs: 7ac7ebef6cd * Update c-ts-mode and c++-ts-mode's verified revision
corfu: bbc205c * Improve docstring

vid-2024-12-25-120202.webm

[amosbird]

It also doesn't work when there are multiple text terminal frames. All frames except the first one that activates the corfu popup will receive this error:

Error in post-command-hook (corfu--post-command): (error "Window is dedicated to ‘xxx’")

The popup's child frame shows garbage content instead.

[minad]

I suggest to report also discuss odd behavior upstream as long as it is not clear that Corfu is at fault. The goal is that TTY and GUI child frames behave as consistently as possible.

[minad]

@amosbird Could you please provide more reproduction steps? I am currently trying to improve Corfu on TTY. Please try the newest commit from the Corfu repo.

[amosbird]

Sure, I’ll try again with the latest commit when I get a moment and see if I can still reproduce it. Thanks!

[amosbird]

I’ve tested again with the latest commit, and I can no longer reproduce all the issues. Everything works fine now on my side.

Thank you very much for your hard work on improving Corfu on TTY, really appreciated!