gtk: sequenced keybind UI indication

[mitchellh]

#2121 was merged and the core now supports it. As noted in the original issue #1420, there are some UX issues with this, so we should investigate UI improvements to make it clear that you are in a sequenced keybind.

[jcollie]

A lot of TUI apps have a status bar at the bottom of the screen where they display any active leaders. The screenshot is from Helix but Neovim is similar IIRC. I personally find that style too unobtrusive. My focus is usually near the center of the screen and so I never really look there. I'd prefer an overlay in the center of the screen similar to the resize overlay, but perhaps semi-transparent so that text behind the screen is still visible. And perhaps a little larger. The position could be configurable for those that want it somewhere else.

[clason]

I would consider this (position) way too intrusive. Recall that Neovim and Helix are TUI applications and can only put characters in the standard font somewhere on the part of the screen they control. On the other hand, Ghostty itself is a GUI and has much more control over what is displayed beyond placement, and I would suggest using that. For example, the URL preview already strikes me as a good compromise, and I would recommend using the same approach here. (If two features use the same UX, this more than doubles its value since you reinforce that "here is where Ghostty puts ephemeral feedback on my actions". And any concern about lack of visibility also applies to the URL preview, so any improvement to it would apply to two features instead of one.)

[jcollie]

Well the beauty of how the resize overlay works is that everyone wins since the position is configurable. I'm fine with the default being at the bottom corner, one more line in my config file won't hurt.

[clason]

The resize is arguably different, since this shows up just when I'm resizing, which is when I'm normally not interested in the contents. Resizing is also a relatively rare occurrence, compared with inputting keystrokes.

[mitchellh]

macOS UI added in #2418. This issue is now a GTK issue.