macOS: a borderless TextField with a background color overhangs its stack by 2pt on each side

[steinybot]

A borderless TextField that has a background color is 4pt wider than the stack it fills on macOS. It overhangs the stack by 2pt on each side, so its border and background draw outside the column.

Repro

import {
  App, Text, TextField, VStack, setPadding, textfieldSetBackgroundColor, textfieldSetBorderless,
  widgetMatchParentWidth, widgetSetBackgroundColor, widgetSetBorderColor, widgetSetBorderWidth,
  widgetSetHeight, widgetSetWidth,
} from "perry/ui"

function field(withBackground: boolean) {
  const f = TextField("", () => {})
  textfieldSetBorderless(f, 1)
  if (withBackground) textfieldSetBackgroundColor(f, 1, 1, 1, 1)
  widgetSetBorderColor(f, 0, 0, 0, 1)
  widgetSetBorderWidth(f, 1)
  return f
}

const a = field(true)
const b = field(false)
const column = VStack(6, [Text("textfieldSetBackgroundColor:"), a, Text("no background color:"), b])
widgetSetWidth(column, 300)
widgetSetBackgroundColor(column, 0.75, 0.87, 1, 1)
for (const f of [a, b]) {
  widgetMatchParentWidth(f)
  widgetSetHeight(f, 32)
}
const root = VStack(0, [column])
setPadding(root, 24, 40, 24, 40)
widgetSetBackgroundColor(root, 1, 1, 1, 1)

App({ title: "TextField width", width: 380, height: 160, body: root })

Expected: both fields are 300pt wide at x = 0.
Actual: field a is 304pt wide at x = -2. Field b is 300pt wide at x = 0. Geisterhand's /widgets?tree=true reports the same frames.

Cause

15158cd "pad text fields as CSS padding does" (#11665) made textfieldSetBackgroundColor paint the layer and call setDrawsBackground(false) on the field. An NSTextField with no bezel, no border, and no background is shaped like a label to AppKit. AppKit gives it a label's alignmentRectInsets: 2pt on the left and the right. Auto Layout pins the alignment rect to the stack, so the frame grows 2pt past each edge. The layer border and the layer background draw on the frame.

Measured with the field pinned to a 590pt stack:

Build	textfieldSetBackgroundColor	drawsBackground	alignmentRectInsets	Frame
main (d368f62)	no	true	0, 0	x 0, w 590
main (d368f62)	yes	false	2, 2	x -2, w 594
main (d368f62)	yes, bezeled	false	0, 0	x 0, w 590
before #11661 (4d4cd17)	yes	true	0, 0	x 0, w 590

native_textfield_cell.rs sets each field's frame by hand, so it does not see the inset.

Proposed fix

Make PerryTextField and PerrySecureTextField return zero alignmentRectInsets. A field must fill the width it is pinned to, whatever it draws. Add a native test that pins a borderless field with a background to a stack and checks that its frame matches the stack's width.