macos: pixel attachments, live theming, and a signed app

The AppKit shell now draws what the core renders, follows the theme without a
relaunch, and builds into something you can hand to someone.

- Pixel attachments. Surface.images was dropped on the floor here, so a PDF
  pane showed nothing at all: native_images is now set, pardes_image_s carries
  the geometry the core already clipped, and PardesView keeps one CGImage per
  (serial, page, revision) so scrolling costs a draw and not a decode. Image
  panes get real pixels instead of the petscii fallback.

- Themes take hold live. pardes_tick never advanced the chrome animation, so
  every tagline kept the previous theme's colours until the next launch and
  the 16 ms re-pump spun for the rest of the session. pardes_theme_bg retires
  the hand-agreed #121212 and drives the window background and the titlebar
  appearance; a theme with no background of its own now gets a transparent
  window over an NSVisualEffectView.

- The cell snaps to whole DEVICE pixels rather than whole points. Monaco
  advances 8.4014pt at 14, so ceiling to 9 spaced every column 7.1% wider than
  the face was drawn for.

- The dial is one notch per 10 degrees instead of 20, and a release keeps
  turning in proportion to how hard it was thrown -- ramping up from zero at
  the floor, so a slow twist coasts not a little but not at all.

- A file dropped on the grid is a click plus Look, so it opens beside the pane
  it was dropped on. No drop concept was added to the core.

- The titlebar follows the focused pane: proxy icon, filename, and the dirty
  dot. File.saved_revision is the watermark that last one needed.

- Config (SPC f c) prints the resolved startup config path.

- build.zig assembles, signs and packages the bundle itself; build-app.sh is
  gone. -Dmacos-identity= takes a Developer ID, macos-dmg makes the image, and
  the icon is Glenda.

1 files changed, 337 insertions, 40 deletions

diff --git a/src/macos/Sources/PardesView.swift b/src/macos/Sources/PardesView.swift
index c7152bd3..3d841a60 100644
--- a/src/macos/Sources/PardesView.swift
+++ b/src/macos/Sources/PardesView.swift
@@ -83,6 +83,12 @@ enum Trackpad {
 let pardesDefaultFG: UInt32 = 0xCC_CC_CC
 let pardesDefaultBG: UInt32 = 0x12_12_12
 
+/// Pinned sRGB for rasterized attachments, so a PDF page's bytes mean the same
+/// thing here as they do in the SDL shell. DeviceRGB is the fallback rather
+/// than a crash: a machine with no sRGB profile is not a reason to stop
+/// drawing pages.
+private let sRGB: CGColorSpace = CGColorSpace(name: CGColorSpace.sRGB) ?? CGColorSpaceCreateDeviceRGB()
+
 // UNVERIFIED: kCTFontAttributeName bridged through NSAttributedString.Key. It is
 // the same string as .font, but spelling the CoreText key means the value stays
 // a CTFont instead of being bridged to NSFont on the way in.
@@ -127,20 +133,31 @@ private enum Face: Int, CaseIterable {
     }
 }
 
-/// `block` means the filled cursor sits on this cell.
+/// A background run that must not be painted at all, so the window's own
+/// backdrop shows through. Outside the 24-bit RGB range, so it can never
+/// collide with a real colour, and distinct from the `.max` the run loop
+/// flushes on.
+let bgClear: UInt32 = 0x0100_0000
+
+/// `block` means the filled cursor sits on this cell. `ground` is what a
+/// DEFAULT background resolves to, and `clearGround` asks for those cells to
+/// come back as `bgClear` instead of a colour.
 private func resolve(
     _ cell: pardes_cell_s,
-    block: Bool
+    block: Bool,
+    ground: UInt32,
+    clearGround: Bool
 ) -> (fg: UInt32, bg: UInt32, alpha: CGFloat, visible: Bool) {
     // The core never painted this cell, which is most of the screen most of the
     // time, so this branch is the one that has to stay cheap.
     if cell.flags & UInt8(PARDES_CELL_DEFAULT) != 0 {
         return block
-            ? (pardesDefaultBG, pardesDefaultFG, 1, false)
-            : (pardesDefaultFG, pardesDefaultBG, 1, false)
+            ? (ground, pardesDefaultFG, 1, false)
+            : (pardesDefaultFG, clearGround ? bgClear : ground, 1, false)
     }
+    let bgDefault = cell.bg == UInt32(PARDES_COLOR_DEFAULT)
     var fg = decodeColor(cell.fg, pardesDefaultFG)
-    var bg = decodeColor(cell.bg, pardesDefaultBG)
+    var bg = decodeColor(cell.bg, ground)
     // The block cursor is a second reverse, so a cell that is already reversed
     // cancels back to normal underneath it. Same rule as emitInstance in
     // src/gui/gui.zig; the two must not drift.
@@ -154,6 +171,9 @@ private func resolve(
     // The other shells scale the channels by 6/10. Over a dark background alpha
     // lands in the same place and costs one blend instead of three multiplies.
     let alpha: CGFloat = cell.attrs & UInt16(PARDES_ATTR_DIM) != 0 ? 0.6 : 1
+    // Only an UNREVERSED default background is the ground. A reverse puts the
+    // text colour there, and text is a real colour that paints.
+    if clearGround && bgDefault && !reverse { bg = bgClear }
     return (fg, bg, alpha, visible)
 }
 
@@ -190,8 +210,16 @@ private struct Metrics {
     /// per face here and never looked up again.
     let asciiGlyphs: [[CGGlyph]]
 
-    init(size: CGFloat, path: String?) {
+    /// Round `v` onto the backing grid: `scale` is the display's
+    /// backingScaleFactor, so at 2x this lands on half-points, which are whole
+    /// device pixels.
+    private static func snap(_ v: CGFloat, _ scale: CGFloat, _ rule: FloatingPointRoundingRule) -> CGFloat {
+        (v * scale).rounded(rule) / scale
+    }
+
+    init(size: CGFloat, path: String?, scale: CGFloat) {
         let face = Metrics.face(size: size, path: path)
+        let scale = max(1, scale)
 
         // UNVERIFIED: CTFontSymbolicTraits member spelling (.traitBold/.traitItalic).
         // A face with no italic cut returns nil here, hence the fallback to `face`.
@@ -200,19 +228,32 @@ private struct Metrics {
         }
         let faces = [face, variant(.traitBold), variant(.traitItalic), variant([.traitBold, .traitItalic])]
 
-        // Whole points, rounded UP. A fractional cell width puts every column
-        // boundary on a fraction of a pixel, and with antialiasing off — which
-        // the background pass needs, or touching fills seam — the rounding
-        // wobbles by a pixel from column to column. On a screen made of tag
-        // bars and selections that stripe is visible. Rounding up rather than
-        // to nearest because down can clip a glyph, and a slightly airy grid is
-        // not a bug.
+        // The grid has to land on WHOLE DEVICE PIXELS, and that is the whole
+        // constraint — a fractional column boundary makes the background pass
+        // (which runs with antialiasing off, or touching fills seam) wobble by
+        // a pixel from column to column, and on a screen made of tag bars and
+        // selections that stripe is visible.
+        //
+        // Whole POINTS is how that used to be spelled, and on a Retina display
+        // it asks for twice what it needs: half a point IS a whole pixel at 2x.
+        // The difference is not academic — Monaco advances 8.4014pt at 14, so
+        // ceiling to 9 spaced every column 7.1% wider than the face was drawn
+        // for, which is loose, washed-out text that reads as bad rendering.
+        // Snapped to the backing grid it is 8.5, i.e. +1.2%.
+        //
+        // Width rounds to NEAREST — a monospace glyph is drawn to fit its own
+        // advance, so the half-pixel either way is slack — while height rounds
+        // UP, because losing a pixel off a descender is clipping.
+        let snap = Metrics.snap
         fonts = faces
-        ascent = CTFontGetAscent(face)
-        cellWidth = max(1, advance(face, 0x4D).rounded(.up))
-        cellHeight = max(1, (CTFontGetAscent(face) + CTFontGetDescent(face) + CTFontGetLeading(face)).rounded(.up))
-        ruleThickness = max(1, CTFontGetUnderlineThickness(face))
-        underlineOffset = CTFontGetUnderlinePosition(face)
+        // The ascent lands on a pixel for a second reason: it is the baseline's
+        // offset inside the cell, so the rules hung off it are whole-pixel
+        // fills rather than one-pixel bars smeared across two rows.
+        ascent = max(1 / scale, snap(CTFontGetAscent(face), scale, .toNearestOrAwayFromZero))
+        cellWidth = max(1 / scale, snap(advance(face, 0x4D), scale, .toNearestOrAwayFromZero))
+        cellHeight = max(1 / scale, snap(CTFontGetAscent(face) + CTFontGetDescent(face) + CTFontGetLeading(face), scale, .up))
+        ruleThickness = max(1 / scale, snap(CTFontGetUnderlineThickness(face), scale, .toNearestOrAwayFromZero))
+        underlineOffset = snap(CTFontGetUnderlinePosition(face), scale, .toNearestOrAwayFromZero)
         asciiGlyphs = faces.map { font in
             var chars = Array(UniChar(0)..<UniChar(128))
             var glyphs = [CGGlyph](repeating: 0, count: 128)
@@ -307,6 +348,31 @@ final class PardesView: NSView {
     private var reportedCols: UInt16 = 0
     private var reportedRows: UInt16 = 0
 
+    /// The backingScaleFactor `metrics` was snapped to, so a move between a
+    /// Retina and a 1x display re-measures the cell instead of leaving the grid
+    /// aligned to the other screen's pixels.
+    private var metricsScale: CGFloat = 2
+
+    /// The active theme's own background, or nil when it declares none — the
+    /// `*_transparent` themes and the curated `dark`. Nil is not a colour to
+    /// substitute but a decision: the ground stops being painted at all, the
+    /// view stops being opaque, and AppDelegate's NSVisualEffectView shows
+    /// through it. Read off pardes_theme_bg() once per pump, which is also
+    /// what makes a `Theme` command take hold without a relaunch.
+    private(set) var themeBG: UInt32? = pardesDefaultBG
+
+    /// Adopt what the core is wearing. Returns whether anything moved, so the
+    /// host only reconfigures the window when it has to.
+    @discardableResult
+    func adoptThemeBG(_ encoded: UInt32) -> Bool {
+        let wanted: UInt32? =
+            encoded == UInt32(PARDES_COLOR_DEFAULT) ? nil : encoded & UInt32(PARDES_COLOR_RGB_MASK)
+        guard wanted != themeBG else { return false }
+        themeBG = wanted
+        needsDisplay = true
+        return true
+    }
+
     // The button a left-stream click actually started with. mouseDown decides
     // it from the fingers on the trackpad, and mouseDragged/mouseUp must use
     // the same one: a press of right followed by a release of left leaves the
@@ -329,7 +395,11 @@ final class PardesView: NSView {
     private var restingFingers: Int = 0
 
     init(fontSize size: CGFloat) {
-        let built = Metrics(size: size, path: nil)
+        // No window yet, so no backing scale to ask for: 2x is the guess every
+        // Mac shipped this decade would give, and viewDidChangeBackingProperties
+        // below re-measures the moment there is a real answer — including the
+        // 1x case, which a bare guess would otherwise leave wrong forever.
+        let built = Metrics(size: size, path: nil, scale: 2)
         metrics = built
         fontSize = size
         fontPath = nil
@@ -340,6 +410,11 @@ final class PardesView: NSView {
 
         runGlyphs.reserveCapacity(256)
         runPositions.reserveCapacity(256)
+        // Files dropped ON the grid. Finder and the Dock already reach the app
+        // through application(_:open:), but that path cannot say WHERE — and
+        // where is the whole difference between "a file opened somewhere" and
+        // acme's "a file opened next to the pane I pointed at".
+        registerForDraggedTypes([.fileURL])
         // Indirect touches are the trackpad's. Without this the touch set is
         // always empty and every click looks like one finger, which is exactly
         // the bug that would make two-finger Look silently never fire.
@@ -371,11 +446,13 @@ final class PardesView: NSView {
     /// system face, and asking for a font that is not wearable leaves the
     /// screen exactly as it was rather than blank.
     private func wear(size: CGFloat, path: String?) {
-        let next = Metrics(size: size, path: path)
+        let scale = window?.backingScaleFactor ?? metricsScale
+        let next = Metrics(size: size, path: path, scale: scale)
         // A CGGlyph is an index into a particular face. Kept across a change
         // it would draw a different character, not a missing one.
         glyphCache.removeAll(keepingCapacity: true)
         metrics = next
+        metricsScale = scale
         fontSize = size
         fontPath = path
         delegate?.pardesViewDidResize(self)
@@ -405,7 +482,10 @@ final class PardesView: NSView {
 
     // Row 0 at the top, so the drawing arithmetic reads like the grid it is.
     override var isFlipped: Bool { true }
-    override var isOpaque: Bool { true }
+    // Opaque only while the theme brings its own background. A transparent
+    // theme has none, and an opaque view over a visual-effect backdrop is a
+    // grey rectangle where the blur should be.
+    override var isOpaque: Bool { themeBG != nil }
     override var acceptsFirstResponder: Bool { true }
     // A click that focuses the window should also land in the grid: this is a
     // text surface, and having to click twice after switching apps is the kind
@@ -428,7 +508,13 @@ final class PardesView: NSView {
         let count = pardes_frame()
         let cols = Int(pardes_frame_cols())
         let rows = Int(pardes_frame_rows())
-        fill(ctx, bounds, pardesDefaultBG, 1)
+        // The ground, from the core rather than from a constant agreed by hand.
+        // A transparent theme has none: CLEAR rather than fill, because AppKit
+        // does not blank a non-opaque view and last frame's pixels would
+        // otherwise pile up on themselves.
+        let ground = themeBG ?? pardesDefaultBG
+        let clearGround = themeBG == nil
+        if clearGround { ctx.clear(bounds) } else { fill(ctx, bounds, ground, 1) }
         guard cols > 0, rows > 0, Int(count) == cols * rows, let cells = pardes_frame_cells() else { return }
 
         // -1 when hidden, which never matches a real cell, so hidden and "bar, so
@@ -446,17 +532,23 @@ final class PardesView: NSView {
             let base = row * cols
             let y = CGFloat(row) * cellHeight
             var start = 0
-            var color = resolve(cells[base], block: blockY == row && blockX == 0).bg
+            var color = resolve(cells[base], block: blockY == row && blockX == 0,
+                                ground: ground, clearGround: clearGround).bg
             for col in 1...cols {
                 // A real color is 24 bits, so .max is a sentinel that cannot
                 // compare equal and therefore always flushes the last run.
                 let next: UInt32 = col == cols
                     ? .max
-                    : resolve(cells[base + col], block: blockY == row && blockX == col).bg
+                    : resolve(cells[base + col], block: blockY == row && blockX == col,
+                              ground: ground, clearGround: clearGround).bg
                 if next == color { continue }
-                fill(ctx, CGRect(x: CGFloat(start) * cellWidth, y: y,
-                                 width: CGFloat(col - start) * cellWidth, height: cellHeight),
-                     color, 1)
+                // bgClear runs are the ground showing through, and the ground is
+                // already clear — painting them would be painting the hole shut.
+                if color != bgClear {
+                    fill(ctx, CGRect(x: CGFloat(start) * cellWidth, y: y,
+                                     width: CGFloat(col - start) * cellWidth, height: cellHeight),
+                         color, 1)
+                }
                 start = col
                 color = next
             }
@@ -467,11 +559,19 @@ final class PardesView: NSView {
         // line mirrored. Un-flip once for the whole glyph pass and convert each
         // baseline into it rather than fighting the text matrix per cell.
         ctx.setShouldAntialias(true)
-        // Every glyph sits at an exact multiple of cellWidth, so letting
-        // CoreText place it on a subpixel would blur a grid that is aligned by
-        // construction. Quantizing keeps the rasterizer's own cache hitting.
+        // Every glyph sits at an exact multiple of cellWidth and the ascent is
+        // whole points (see Metrics), so every baseline is already on a pixel:
+        // letting CoreText place a glyph on a subpixel would blur a grid that
+        // is aligned by construction. Quantizing keeps the rasterizer's own
+        // cache hitting.
         ctx.setShouldSubpixelPositionFonts(false)
         ctx.setShouldSubpixelQuantizeFonts(true)
+        // Grayscale antialiasing, never LCD subpixel. Smoothing needs to know
+        // the colour behind the glyph, which over a transparent theme's
+        // backdrop it cannot — the result is coloured fringing that reads as
+        // blur. macOS has defaulted this off since 10.14, but the user can turn
+        // it back on globally and it is not their call to make for this grid.
+        ctx.setShouldSmoothFonts(false)
         ctx.saveGState()
         ctx.textMatrix = .identity
         ctx.translateBy(x: 0, y: bounds.height)
@@ -484,12 +584,20 @@ final class PardesView: NSView {
         }
         ctx.restoreGState()
 
+        // Pixel attachments over the grid: rasterized PDF pages, and image
+        // panes' own pixels. After the glyphs, the way the SDL shell draws them
+        // after its cells — a PDF pane's cells are blank, so the order only
+        // matters for the tag row an attachment must never reach, and the clip
+        // below is what keeps it off.
+        drawImages(ctx)
+
         if bar {
             let x = Int(pardes_cursor_x()), y = Int(pardes_cursor_y())
             if x >= 0, y >= 0, x < cols, y < rows {
                 // gui.zig paints U+258F here. A rect is the same picture without
                 // asking the font for a glyph it may not carry.
-                let fg = resolve(cells[y * cols + x], block: false).fg
+                let fg = resolve(cells[y * cols + x], block: false,
+                                 ground: themeBG ?? pardesDefaultBG, clearGround: false).fg
                 ctx.setShouldAntialias(false)
                 fill(ctx, CGRect(x: CGFloat(x) * cellWidth, y: CGFloat(y) * cellHeight,
                                  width: max(1, (cellWidth / 8).rounded(.up)), height: cellHeight), fg, 1)
@@ -497,6 +605,104 @@ final class PardesView: NSView {
         }
     }
 
+    /// What identifies a decoded raster: the pane's lifetime, the page, and the
+    /// generation MuPDF last rendered. Panning, zooming to fit and scrolling
+    /// deliberately move none of them, so the CGImage survives all three.
+    private struct ImageKey: Hashable {
+        let serial: UInt32
+        let page: UInt32
+        let revision: UInt32
+    }
+
+    /// Rasterized attachments, decoded once each. The bytes the core lends are
+    /// only valid until the next `pardes_frame`, so the CGImage owns a COPY —
+    /// which is exactly why the cache has to be keyed well enough that the copy
+    /// happens when the pixels change and never on an ordinary scroll.
+    private var imageCache: [ImageKey: CGImage] = [:]
+
+    private func drawImages(_ ctx: CGContext) {
+        let count = Int(pardes_frame_images())
+        guard count > 0, let list = pardes_frame_image_list() else {
+            // Nothing on screen owns pixels any more: the pages a closed pane
+            // rendered would otherwise sit in here for the rest of the session.
+            if !imageCache.isEmpty { imageCache.removeAll(keepingCapacity: true) }
+            return
+        }
+
+        // The core computed every rectangle in PHYSICAL pixels, because that is
+        // what pardes_resize handed it. The view draws in points.
+        let scale = max(1, metricsScale)
+        var live = Set<ImageKey>()
+        live.reserveCapacity(count)
+
+        ctx.setShouldAntialias(true)
+        for i in 0..<count {
+            let place = list[i]
+            let key = ImageKey(serial: place.serial, page: place.page, revision: place.revision)
+            live.insert(key)
+            guard let full = image(for: place, key: key) else { continue }
+            guard let crop = full.cropping(to: CGRect(
+                x: Int(place.src_x), y: Int(place.src_y),
+                width: Int(place.src_w), height: Int(place.src_h)))
+            else { continue }
+
+            // The body is the rectangle nothing may paint past. The core has
+            // already clipped the geometry to the viewport, but a tagline is
+            // not the viewport — a page one pixel too tall would sit on it.
+            let body = CGRect(
+                x: CGFloat(place.cell_x) * cellWidth, y: CGFloat(place.cell_y) * cellHeight,
+                width: CGFloat(place.cell_w) * cellWidth, height: CGFloat(place.cell_h) * cellHeight)
+            let dst = CGRect(
+                x: body.minX + CGFloat(place.dst_x) / scale,
+                y: body.minY + (CGFloat(place.dst_y) + CGFloat(place.offset_y)) / scale,
+                width: CGFloat(place.dst_w) / scale,
+                height: CGFloat(place.dst_h) / scale)
+
+            ctx.saveGState()
+            ctx.clip(to: body)
+            // isFlipped gives us a y-down CTM and CGImage draws +y up, so a
+            // plain ctx.draw would land every page upside down. Flip about the
+            // destination rather than about the view, so the arithmetic above
+            // stays in the grid's own coordinates.
+            ctx.translateBy(x: dst.minX, y: dst.maxY)
+            ctx.scaleBy(x: 1, y: -1)
+            // A page is resampled whenever fit or zoom disagrees with the
+            // raster MuPDF last produced; nearest-neighbour text is unreadable.
+            ctx.interpolationQuality = .high
+            ctx.draw(crop, in: CGRect(x: 0, y: 0, width: dst.width, height: dst.height))
+            ctx.restoreGState()
+        }
+
+        // Evict what this frame did not place. Scrolling a document past a page
+        // is the common case, and holding every page a session ever showed is
+        // how a PDF viewer ends up owning a gigabyte of decoded bitmaps.
+        if imageCache.count > live.count {
+            imageCache = imageCache.filter { live.contains($0.key) }
+        }
+    }
+
+    /// The decoded raster for one attachment, made once per generation.
+    private func image(for place: pardes_image_s, key: ImageKey) -> CGImage? {
+        if let cached = imageCache[key] { return cached }
+        let bytes = Int(place.iw) * Int(place.ih) * 4
+        guard bytes > 0, let rgba = place.rgba else { return nil }
+        // Copied, not referenced: the core lends these bytes until the next
+        // pardes_frame and this image outlives many of them.
+        guard let data = CFDataCreate(nil, rgba, bytes),
+              let provider = CGDataProvider(data: data)
+        else { return nil }
+        // Straight alpha, R,G,B,A in memory — the same bytes the SDL shell
+        // uploads as R8G8B8A8_UNORM and blends with ONE_MINUS_SRC_ALPHA.
+        let made = CGImage(
+            width: Int(place.iw), height: Int(place.ih),
+            bitsPerComponent: 8, bitsPerPixel: 32, bytesPerRow: Int(place.iw) * 4,
+            space: sRGB,
+            bitmapInfo: CGBitmapInfo(rawValue: CGImageAlphaInfo.last.rawValue | CGBitmapInfo.byteOrder32Big.rawValue),
+            provider: provider, decode: nil, shouldInterpolate: true, intent: .defaultIntent)
+        if let made { imageCache[key] = made }
+        return made
+    }
+
     /// One row of glyphs, batched. Consecutive cells that share a face and a
     /// colour go to CoreText as a single call with a position array: a row of
     /// plain text is then one draw instead of eighty, which is the difference
@@ -526,7 +732,10 @@ final class PardesView: NSView {
         for col in 0..<cols {
             let cell = cells[base + col]
             if cell.flags & UInt8(PARDES_CELL_DEFAULT) != 0 { continue }
-            let style = resolve(cell, block: blockCol == col)
+            // clearGround: false — this pass only reads `fg`, and a glyph is
+            // never the hole in the ground.
+            let style = resolve(cell, block: blockCol == col,
+                                ground: themeBG ?? pardesDefaultBG, clearGround: false)
             let x = CGFloat(col) * cellWidth
 
             // Rules before the glyph, and independent of it: an underlined space
@@ -621,7 +830,9 @@ final class PardesView: NSView {
             }
         }
         if cell.attrs & UInt16(PARDES_ATTR_STRIKETHROUGH) != 0 {
-            fill(ctx, CGRect(x: x, y: baseline + metrics.ascent * 0.3, width: cellWidth, height: metrics.ruleThickness),
+            // Rounded like every other rule offset: a third of the ascent is a
+            // fraction, and a fractional one-pixel bar is a two-pixel smear.
+            fill(ctx, CGRect(x: x, y: baseline + (metrics.ascent * 0.3).rounded(), width: cellWidth, height: metrics.ruleThickness),
                  style.fg, style.alpha)
         }
     }
@@ -704,6 +915,15 @@ final class PardesView: NSView {
         fed()
     }
 
+    /// The fingers came off the trackpad. A post-decode entry point of its own
+    /// so the e2e harness can throw the dial: NSEvent phases have no public
+    /// constructor, and a fling nothing can synthesize is a fling nothing can
+    /// assert.
+    func rotateEnd() {
+        pardes_rotate_end()
+        fed()
+    }
+
     // MARK: - keyboard
 
     override func keyDown(with event: NSEvent) {
@@ -903,13 +1123,22 @@ final class PardesView: NSView {
     /// Two fingers twisted on the trackpad are the search-step keys: clockwise
     /// walks forward through the matches, counterclockwise back. It is a dial,
     /// and n/N is what a dial over a list of hits means. libpardes owns the
-    /// quantizing, exactly as it does for scroll.
+    /// quantizing and the momentum, exactly as it owns the scroll accumulator.
+    ///
+    /// AppKit gives rotation no momentum phase of its own — `momentumPhase` is
+    /// scroll's alone — so the fling is measured from the release speed on the
+    /// Zig side rather than handed to us. All this has to get right is telling
+    /// it where the gesture starts and stops.
     override func rotate(with event: NSEvent) {
         trace("rotate: degrees=\(event.rotation) phase=\(event.phase.rawValue)")
         // A gesture starting drops whatever the last one left banked, so the
-        // first degree of a new twist cannot inherit a nearly-complete notch.
+        // first degree of a new twist cannot inherit a nearly-complete notch —
+        // and stops a fling still coasting, because a finger back down is how
+        // a hand catches a dial.
         if event.phase == .began { pardes_rotate(0) }
         rotate(degrees: CGFloat(event.rotation))
+        // .cancelled too: a gesture the system took away should not fling.
+        if event.phase == .ended || event.phase == .cancelled { rotateEnd() }
     }
 
     /// What the trackpad actually delivered, under PARDES_LOG — the same
@@ -946,6 +1175,65 @@ final class PardesView: NSView {
         return GridPoint(col: UInt16(col), row: UInt16(row))
     }
 
+    // MARK: - files dropped on the grid
+
+    /// A drop is a CLICK followed by `Look`, and that is the whole definition.
+    ///
+    /// The core has no notion of a drop and is not being given one: the pointer
+    /// lands where it landed, which focuses that pane exactly as a left click
+    /// there would, and then the ordinary `Look` builtin runs in it — so the
+    /// document opens beside the pane you pointed at rather than beside
+    /// whichever one happened to be focused. Drop on a tag and you clicked a
+    /// tag; there is no case to special-case, and nothing here the hand could
+    /// not have done itself.
+    override func draggingEntered(_ sender: NSDraggingInfo) -> NSDragOperation {
+        // AppKit reuses this answer for draggingUpdated when that is not
+        // implemented, so the cursor stays right for the whole drag.
+        droppedFiles(sender).isEmpty ? [] : .copy
+    }
+
+    override func performDragOperation(_ sender: NSDraggingInfo) -> Bool {
+        let paths = droppedFiles(sender)
+        guard !paths.isEmpty else { return false }
+        drop(paths, at: cellAt(convert(sender.draggingLocation, from: nil)))
+        return true
+    }
+
+    /// The drop, decoded: paths and a cell, nothing AppKit left in it.
+    ///
+    /// Split out for the reason every gesture here is — `NSDraggingInfo` is a
+    /// protocol with a dozen members and no public conformer, so a test that
+    /// had to build one would be testing its own stub. The decision lives one
+    /// call below the event, and `drop` in test/macos_e2e.swift drives exactly
+    /// this.
+    ///
+    /// A nil cell is a drop before the first frame, which has no grid to point
+    /// at: the files still open, they just open where focus already was.
+    func drop(_ paths: [String], at target: GridPoint?) {
+        if let target {
+            press(PARDES_MOUSE_LEFT, at: target)
+            release(PARDES_MOUSE_LEFT, at: target)
+        }
+        // Whole tail, unquoted: executeBuiltinLine takes everything after the
+        // first word as the argument, so a path with spaces in it needs no
+        // escaping and would in fact break under any.
+        for path in paths {
+            let line = "Look \(path)"
+            line.withCString { pardes_command($0, line.utf8.count) }
+        }
+        fed()
+    }
+
+    /// File paths on the drag pasteboard, in order. Empty for anything else,
+    /// which is also how draggingEntered decides whether to accept at all.
+    private func droppedFiles(_ sender: NSDraggingInfo) -> [String] {
+        let options: [NSPasteboard.ReadingOptionKey: Any] = [.urlReadingFileURLsOnly: true]
+        guard let urls = sender.draggingPasteboard.readObjects(
+            forClasses: [NSURL.self], options: options) as? [URL]
+        else { return [] }
+        return urls.map(\.path)
+    }
+
     // MARK: - geometry
 
     override func updateTrackingAreas() {
@@ -979,13 +1267,22 @@ final class PardesView: NSView {
 
     // Dragging the window between a Retina display and a 1x one changes the
     // backing scale without moving a single bound, so setFrameSize above never
-    // fires and the physical cell metrics the core uses to place PDF pages stay
-    // at the old scale forever. This is the only notification of it. (Ghostty
-    // hooks the same one, and additionally re-fires from the window's
-    // didChangeScreen notification, which AppKit does not always pair with it.)
+    // fires. This is the only notification of it. (Ghostty hooks the same one,
+    // and additionally re-fires from the window's didChangeScreen
+    // notification, which AppKit does not always pair with it.)
+    //
+    // TWO things depend on the scale: the physical cell metrics the core uses
+    // to place PDF pages, and the cell itself, which is snapped to whole
+    // DEVICE pixels (see Metrics) and is therefore aligned to the display it
+    // was measured on. Re-measuring reports the resize on its own, so the
+    // delegate call is the else-branch and not an extra one.
     override func viewDidChangeBackingProperties() {
         super.viewDidChangeBackingProperties()
-        delegate?.pardesViewDidResize(self)
+        if let scale = window?.backingScaleFactor, scale != metricsScale {
+            wear(size: fontSize, path: fontPath)
+        } else {
+            delegate?.pardesViewDidResize(self)
+        }
     }
 }