[feature request] add an option to enable tmux popup window borders

[aead]

Checklist

• I have read through the manual page (man fzf)
• I have searched through the existing issues
• For bug reports, I have checked if the bug is reproducible in the latest version of fzf

Output of fzf --version

0.57.0

OS

• Linux
• macOS
• Windows
• Etc.

Shell

• bash
• zsh
• fish

Problem / Steps to reproduce

Currently, the tmux popup window is launched with:

fzf/src/tmux.go

Line 36 in fb3bf6c

tmuxArgs := []string{"display-popup", "-E", "-B", "-d", dir}

The -B flag disables the popup window border. This seems to be a good default. However, there are situations where it would be nice if the popup window had a border. For example, I have a fzf command listing git-log entries and a key binding that executes a pager for viewing the commit message or entire commit (including diff).

Without a popup border, the pager output isn't visually separated from the background and harder to read.

vs.

If customizing this should be supported, I see two ways to do this:

• Allow tmux flags in the list of elements passed via --tmux[=[center|top|bottom|left|right][,SIZE[%]][,SIZE[%]]]
  For example, --tmux[=[center|top|bottom|left|right][,SIZE[%]][,SIZE[%]],[-B],[-E],[-d DIR]]. If no flags are
  specified, the current list of default flags is applied.
• Add a new flag --tmux-flags that contains a list of flags passed to tmux popup. Again, if not specified, the current
  list of default flags is applied.

[aead]

Another variation of the above proposed solution would be extending the --tmux flag to:

--tmux[=[center|top|bottom|left|right][,SIZE[%]][,SIZE[%]][,border]]

which would wrap the tmux interface and not expose tmux flag names as part of the fzf interface.

[junegunn]

There is an ongoing discussion where we're trying to abstract --tmux into --popup and support both tmux, zellij, and any alternative that would allow you to open a popup window. See #4145

So considering that, I'd like to avoid leaking any tmux-specific options to the users, so I prefer your last suggestion, but the word border alone can easily be misunderstood. We should be more specific, maybe something like border-native.

[junegunn]

Somewhat related: #4155

[aead]

So considering that, I'd like to avoid leaking any tmux-specific options to the users, so I prefer your last suggestion, but the word border alone can easily be misunderstood. We should be more specific, maybe something like border-native.

Sounds reasonable to me. With a more generic --popup flag in mind the options should be generic as well and def. not leak popup-implementation specifics into the fzf interface. I can send a PR for the current --tmux flag. I assume it has to remain for backward compatibility anyway but becomes an alias for --popup eventually.