Add opt-in ghostel-line-spacing for padded terminal rows#536
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New defcustom applies buffer-local line-spacing (integer pixels or a float fraction of the line height) in ghostel buffers. The grid sizing already follows the taller rows through window-screen-lines; the reported cell pixel height now includes the spacing so XTWINOPS replies and kitty placement math match rendered rows, and the custom setter resizes open terminals immediately instead of leaving a stale grid until the next window resize. Default 0 keeps current behavior. Non-zero values leave gaps in everything that tiles vertically (box-drawing, block characters, kitty image rows) since Emacs draws no content into the spacing strip; the docstring and README row document this.
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Follow-up to #535 / discussion #534: an escape hatch for users who want padded terminal lines despite the tradeoffs.
New
ghostel-line-spacingdefcustom (integer = extra pixels below each row, float = fraction of the line height; same semantics as Emacs'line-spacing, cons form unsupported since the sizing math cannot see it). Default0is a behavioral no-op — this changes nothing for existing setups.What it does when set:
line-spacingat mode init. Grid sizing needs no changes —window-screen-linesdivides bydefault-line-height, which includes buffer-local spacing.ghostel--reported-cell-heightnow includes the spacing (newghostel--cell-heighthelper), so XTWINOPSCSI 14/16 treplies and kitty placementgrid_rowsmath match the rendered rows instead of under-reporting by the spacing ratio.:setupdates live terminals immediately (buffer-local value + forcedghostel--adjust-sizeper window), avoiding the stale grid that a rawsetqwould leave until the next window resize (nothing fires on line-height-only changes — the window's pixel size is unchanged).Known cosmetic cost, documented in the docstring and README table row: Emacs draws no content into the spacing strip, so non-zero values leave horizontal gaps in everything that tiles vertically — box-drawing borders, block characters, half-block image output, and kitty image rows. Kitty tiling is deliberately left unchanged: buffer
line-spacingis applied per glyph in redisplay, so image rows pad below their slice exactly like text rows (uniform cell-height rows); no newlineline-heightproperty can make the slices flush (numeric,(HEIGHT TOTAL), andtforms all verified ineffective against per-glyph spacing).Verified live (GUI Emacs): integer and float spacing size the grid and SIGWINCH correctly (33→27 rows at 3px, 33→30 at 0.1);
customize-set-variablemid-session resizes 33→23 rows and back with zero vscroll;CSI 14 treports exactly rows × reported cell height; kitty images render uniform cell-height rows at spacing 6 and are pixel-identical to main at spacing 0; box-drawing shows only the documented gaps. Includes three batch ERT tests (mode init, live:setpropagation, cell-height arithmetic).