Add pad-extend option (like Ghostty) #2272

Adds a new boolean option pad-extend that extends edge cell
background colors into the terminal padding/margins, similar to
Ghostty's behavior. When enabled, the left/right margins use the
background color of the adjacent edge cells, and top/bottom margins
use the colors of the first/last row cells.

pad-extend = yes

FWIW, I've been running this for quite some time, no issues so far.

foot version: 1.26.1-1-g83794ec2 (Mar 18 2026, branch 'HEAD') +pgo +ime +graphemes +toplevel-tag -blur -assertions

Found a bug where the "pad-extend" border gets an alpha channel even though I'm in "default" alpha-mode.

Might want to copy the alpha component from the function I wrote here into your cell_bg_color.
#2310/files

(Screenshot taken with 25x25 padding)

image

@cow_enthusiast thanks for choosing this PR, i feel honored; and for catching the alpha-issue (i never use transparencies whereever i can turn it off, so i would have never ever catched that!). Should be fixed: since these edge fills are guarded by != _bg, the cells can't have a default background, so they only need to be opaque unless alpha-mode=all.

@dnkl I also tryed to gate the edge-extend to full-height renders, so the partial scroll-restore paths stop redoing margin work the per-frame re-render already covers, to keep it lighter on damage.

One deliberate scope call I want to flag:
cell_bg_color() resolves the plain background, so a reverse-video edge cell currently bleeds its background instead of its visible color into the padding. The minimal fix could be ~8 lines, to return the foreground when the cell is reversed:

 if (unlikely(cell->attrs.reverse)) {
 switch (cell->attrs.fg_src) {
 case COLOR_RGB: return cell->attrs.fg;
 case COLOR_BASE16:
 case COLOR_BASE256: return term->colors.table[cell->attrs.fg];
 case COLOR_DEFAULT: return term->reverse ? term->colors.bg :
 term->colors.fg;
 }
 }

I left it out to keep the diff minimal — and i'm flagging it not because i ever actually noticed it, but to respect what #2310 (closed in favor of this) would have covered. Selected edge cells and the reverse-video + alpha-mode=all combo are the remaining gaps, both niche/transient, so I'd punt on those unless you disagree.
🐮's cell_effective_bg() is the full-coverage version if we ever want all of it.