[Feature] Sticky headers about streampager

[markbt]

This is a great idea. I already had a similar idea where a static banner could be added to the file which gets shown at the top. I was anticipating using that for showing titles, which would be particularly useful for controlled mode.

This is subtly different: my proposed banners would be entirely static, whereas (I assume) these sticky header lines would scroll left and right with the content. You could also use it, e.g. for showing large tables of SQL output, where you can make the first 3 lines sticky and only the data scrolls.

I'm not sure custom ANSI sequences would work so well, as I'm not sure what they'd mean anywhere but at the top. Instead I propose the following (defining some terms while I'm at it):

• The header is an initial number of file lines that are always shown at the top of the file-view portion of the screen. They scroll left and right with the rest of the content when wrapping is disabled, but do not scroll up and down.

  • The header can be controlled by keyboard shortcuts, increasing or decreasing the number of lines.

  • The header's initial size can be set by command line flags (on a per-stream basis). For example: git log -p | sp --header-lines=3

  • The header's initial size can also be set using parameters to the Pager when using streampager as a library.

• The banner is a set of lines that is always shown at the top of the screen above the file content. It can only be set when using streampager as a library.

[markbt]

If you think an escape sequence might be useful (e.g. because you want a template to print it out to get the right line), then perhaps we could use an OSC sequence. Something like:

ESC ] streampager ; set-end-of-header BEL

This should be ignored by terminals, I believe.

In termwiz it gets parsed as OperatingSystemCommand::Unspecified(["streampager", "set-end-of-header"]) which should make it easy enough to parse out in line.rs.

[martinvonz]

Once this feature is in, it would be nice to also have keyboard shortcuts to scroll between the headers. I often review a series of patches by running hg log -pvr <some revset>. It would be nice to be able to press some key(s) to get the next/previous changeset/file header at the top of the screen.