Center glyphs vertically when lineSpacing > 1

[Coiggahou2002]

What

When lineSpacing > 1, vertically center the glyph within the (now taller) cell, splitting the extra space above and below — instead of leaving it all above the text.

Why

lineSpacing scales the cell height in computeFontDimensions():

let cellHeight = ceil((lineAscent + lineDescent + lineLeading) * _lineSpacing)

but drawTerminalContents() still positions the baseline with the un-scaled offset:

let yOffset = ceil(lineDescent + lineLeading)

Since the row is drawn from the bottom up, the glyph baseline stays pinned to the bottom of the cell and all the added spacing ends up above the text. At larger values (e.g. 1.4–1.6) the lines look bottom-heavy — the gap sits entirely between a line and the line above it, not balanced around each line.

Change

Compute the extra height introduced by lineSpacing and split it evenly:

let extra = max(0, cellDimension.height - ceil(lineAscent + lineDescent + lineLeading))
let yOffset = ceil(lineDescent + lineLeading + extra / 2)

• No behavior change at lineSpacing == 1.0 (extra == 0).
• Backgrounds, selection and cursor already fill the full cell height, so only the glyph position changes.
• Builds clean (swift build).

This matches how iTerm2 and WezTerm distribute their line spacing.

[migueldeicaza]

I like the idea in principle, but this is not in sync with the caretView, we recently had to address that divergence, should be easy to test if you use a text editor and go to that line, the cursor is likely out of place,

This also does not cover metal.

Lastly, double-height characters might need to be addressed - check glyphSlotFit for details.