nandi/jolt-native · 0b92f67

⑂ 0b92f67

Answer the rest of what a client asks its window

The other seven glimmer-vidya calls frq makes: the title, the display, the
browser, the clipboard, the picture chooser and asking to close. None of it
is drawing and none of it belongs in a widget, but a client cannot do any of
it either — the window handle lives in the toolkit, so the questions come
here.

jvui.host holds that handle and answers a harmless nothing before a window
exists: no title to set, no display to measure, no clipboard to read.
jvui's own tests run with no window and should not have to guard every
call, and neither should a client during startup.

quit! is a REQUEST, not an exit. The loop owns teardown — textures,
painter, fonts, the window — and tearing any of it down from under a walk
that is still running is how closing turns into crashing.

screen-size is the display's USABLE bounds rather than its raw ones: a
window sized to the whole display is one with its bottom edge under a
taskbar.

pick-image! answers false and picked-image! nil, and those are the correct
answers rather than gaps. The chooser exists so a phone can hand back a
grant for one picture without the app holding a permission over all of
them; a desktop has no such thing, browsing the filesystem is the answer
there, and glimmer-vidya says exactly the same thing on a desktop. An
Android jvui would replace them.

clipboard-image-png! asks for image/png only. A clipboard may hold a bitmap
or a TIFF, and converting those means an encoder in here — a lot of toolkit
for a paste, when every desktop that puts a picture on a clipboard this
decade puts a PNG there too.

Tested against a real window under SDL's dummy video driver, including the
before-a-window answers and that quit! actually stops the loop early. Not
open-url!: succeeding would open a browser on somebody's desktop and
failing would only say this container has none.

That is twelve of twelve. frq's whole glimmer-vidya surface now exists here.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nandi committed 2026-09-10T05:39:33-07:00 Browse files

0b92f67 parent: 32fae8d

modified glimmer-backends/glimmer-jvui/src/glimmer_jvui/core.clj +22 -1 Viewed

@@ -40,7 +40,8 @@
40	40	[jvui.core :as c]
41	41	[jvui.theme :as theme]
42	42	[jvui.widgets :as w]
43		- [jvui.frames :as frames]))
	43	+ [jvui.frames :as frames]
	44	+ [jvui.host :as host]))
44	45
45	46	;; --- the retained tree -------------------------------------------------------
46	47
@@ -326,3 +327,23 @@
326	327	"Every feed with a picture."
327	328	[]
328	329	(frames/keys*))
	330	+
	331	+;; --- the platform -----------------------------------------------------------
	332	+;; The rest of what glimmer-vidya answers, so a client can ask its backend
	333	+;; about the window it is in without knowing which backend that is. Thin on
	334	+;; purpose: every one of these is jvui.host, and the indirection exists so
	335	+;; the client requires one namespace rather than two.
	336	+
	337	+(def set-title! host/set-title!)
	338	+(def window-width host/window-width)
	339	+(def screen-size host/screen-size)
	340	+(def quit! host/quit!)
	341	+(def open-url! host/open-url!)
	342	+(def clipboard-image-png! host/clipboard-image-png!)
	343	+
	344	+;; False and nil on a desktop, which is the right answer rather than a gap:
	345	+;; the chooser exists so a phone can hand back a grant for one picture, and
	346	+;; a caller reads the false and offers a file browser instead. glimmer-vidya
	347	+;; says the same thing here.
	348	+(def pick-image! host/pick-image!)
	349	+(def picked-image! host/picked-image!)

              [jvui.core :as c]
              [jvui.theme :as theme]
              [jvui.widgets :as w]
--            [jvui.frames :as frames]))
++            [jvui.frames :as frames]
++            [jvui.host :as host]))
  ;; --- the retained tree -------------------------------------------------------
    "Every feed with a picture."
    []
    (frames/keys*))
++
++;; --- the platform -----------------------------------------------------------
++;; The rest of what glimmer-vidya answers, so a client can ask its backend
++;; about the window it is in without knowing which backend that is. Thin on
++;; purpose: every one of these is jvui.host, and the indirection exists so
++;; the client requires one namespace rather than two.
++
++(def set-title!          host/set-title!)
++(def window-width        host/window-width)
++(def screen-size         host/screen-size)
++(def quit!               host/quit!)
++(def open-url!           host/open-url!)
++(def clipboard-image-png! host/clipboard-image-png!)
++
++;; False and nil on a desktop, which is the right answer rather than a gap:
++;; the chooser exists so a phone can hand back a grant for one picture, and
++;; a caller reads the false and offers a file browser instead. glimmer-vidya
++;; says the same thing here.
++(def pick-image!         host/pick-image!)
++(def picked-image!       host/picked-image!)

modified jvui/src/jvui/app.clj +8 -0 Viewed

@@ -8,6 +8,7 @@
8	8	[jvui.font :as font]
9	9	[jvui.paint :as paint]
10	10	[jvui.frames :as frames]
	11	+ [jvui.host :as host]
11	12	[jvui.theme :as theme]
12	13	[jvui.core :as c]))
13	14
@@ -73,6 +74,7 @@
73	74	;; into. Anything that arrives before this is dropped rather than
74	75	;; queued — see jvui.frames.
75	76	(frames/install-renderer! renderer)
	77	+ (host/install! window)
76	78	(sdl/start-text-input! window)
77	79	(try
78	80	(loop [n 0]
@@ -97,11 +99,17 @@
97	99	(sdl/present! renderer)
98	100	(sdl/delay-ms! 16)
99	101	(when-not (or (:quit? @ctx)
	102	+ ;; A client asking to close — see jvui.host/quit!.
	103	+ ;; Checked here so teardown stays the loop's, which
	104	+ ;; is the only thing that knows what is still being
	105	+ ;; walked.
	106	+ (host/quit-requested?)
100	107	(and frames (>= (inc n) frames))
101	108	(and deadline (> (System/currentTimeMillis) deadline)))
102	109	(recur (inc n)))))
103	110	(finally
104	111	(frames/clear!)
	112	+ (host/clear!)
105	113	(paint/close! painter)
106	114	(font/close! fonts)
107	115	(sdl/close! win))))))

              [jvui.font :as font]
              [jvui.paint :as paint]
              [jvui.frames :as frames]
++            [jvui.host :as host]
              [jvui.theme :as theme]
              [jvui.core :as c]))
        ;; into. Anything that arrives before this is dropped rather than
        ;; queued — see jvui.frames.
        (frames/install-renderer! renderer)
++      (host/install! window)
        (sdl/start-text-input! window)
        (try
          (loop [n 0]
              (sdl/present! renderer)
              (sdl/delay-ms! 16)
              (when-not (or (:quit? @ctx)
++                          ;; A client asking to close — see jvui.host/quit!.
++                          ;; Checked here so teardown stays the loop's, which
++                          ;; is the only thing that knows what is still being
++                          ;; walked.
++                          (host/quit-requested?)
                            (and frames (>= (inc n) frames))
                            (and deadline (> (System/currentTimeMillis) deadline)))
                (recur (inc n)))))
          (finally
            (frames/clear!)
++          (host/clear!)
            (paint/close! painter)
            (font/close! fonts)
            (sdl/close! win))))))

added jvui/src/jvui/host.clj +112 -0 Viewed

new file mode 100644
@@ -0,0 +1,112 @@
	1	+(ns jvui.host
	2	+ "The window as a thing in an operating system, rather than a thing to draw in.
	3	+
	4	+ Its title, the display it is on, the browser, the clipboard, and asking it
	5	+ to close. None of this is painting and none of it belongs in a widget, but
	6	+ a client cannot do any of it either — the window handle lives here, so the
	7	+ questions come here too.
	8	+
	9	+ Everything answers a harmless nothing before a window exists: no title to
	10	+ set, no display to measure, no clipboard to read. A client that runs
	11	+ headless — jvui's own tests do — should not have to guard every call."
	12	+ (:require [jvui.sdl :as sdl]))
	13	+
	14	+(defonce ^:private win (atom nil))
	15	+(defonce ^:private quit? (atom false))
	16	+
	17	+(defn install!
	18	+ "Called by the frame loop when a window opens."
	19	+ [window]
	20	+ (reset! win window)
	21	+ (reset! quit? false))
	22	+
	23	+(defn clear! [] (reset! win nil) nil)
	24	+
	25	+(defn window
	26	+ "The open window, or nil."
	27	+ []
	28	+ @win)
	29	+
	30	+;; --- closing -----------------------------------------------------------------
	31	+
	32	+(defn quit!
	33	+ "Ask the frame loop to stop after the current frame.
	34	+
	35	+ A request rather than an exit: the loop owns teardown — textures, painter,
	36	+ fonts, the window itself — and tearing any of it down from under a walk
	37	+ that is still running is how a close turns into a crash."
	38	+ []
	39	+ (reset! quit? true)
	40	+ nil)
	41	+
	42	+(defn quit-requested? [] @quit?)
	43	+
	44	+;; --- the window --------------------------------------------------------------
	45	+
	46	+(defn set-title! [s]
	47	+ (when-let [w @win] (sdl/set-window-title! w (str s)))
	48	+ nil)
	49	+
	50	+(defn window-size
	51	+ "[w h] in window units, or nil with no window."
	52	+ []
	53	+ (when-let [w @win] (sdl/window-size w)))
	54	+
	55	+(defn window-width
	56	+ "Width in window units, or 0 with no window — a caller dividing by it wants
	57	+ a number, and zero is the honest one."
	58	+ []
	59	+ (or (first (window-size)) 0))
	60	+
	61	+(defn screen-size
	62	+ "[w h] of the display this window is on, or nil with no window."
	63	+ []
	64	+ (when-let [w @win] (sdl/display-bounds w)))
	65	+
	66	+;; --- the world outside -------------------------------------------------------
	67	+
	68	+(defn open-url!
	69	+ "Hand a URL to whatever the desktop opens URLs with."
	70	+ [url]
	71	+ (boolean (and (string? url) (seq url) (sdl/open-url! url))))
	72	+
	73	+(defn clipboard-image-png!
	74	+ "Write the clipboard's picture to `path` as PNG; true when there was one.
	75	+
	76	+ Only image/png is asked for. A clipboard may also hold a bitmap or a TIFF
	77	+ and converting those would mean an encoder in here, which is a lot of
	78	+ toolkit for a paste — and every desktop that puts a picture on a clipboard
	79	+ this decade puts a PNG there too."
	80	+ [path]
	81	+ (boolean
	82	+ (when (sdl/has-clipboard-data "image/png")
	83	+ (when-let [[p n] (sdl/clipboard-data "image/png")]
	84	+ (try
	85	+ (let [bs (byte-array n)]
	86	+ (jolt.ffi/read-array p :byte bs)
	87	+ (java.nio.file.Files/write
	88	+ (java.nio.file.Path/of (str path) (into-array String []))
	89	+ bs
	90	+ (into-array java.nio.file.OpenOption []))
	91	+ true)
	92	+ (finally (sdl/sdl-free! p)))))))
	93	+
	94	+;; --- the picture chooser ------------------------------------------------------
	95	+;; There is not one, and that is the same answer glimmer-vidya gives on a
	96	+;; desktop. The chooser exists so a phone can hand back a grant for one
	97	+;; picture without the app holding a permission over all of them; a desktop
	98	+;; has no such thing and browsing the filesystem is the answer there.
	99	+;;
	100	+;; So these are not stubs waiting to be filled in on this backend — they are
	101	+;; the correct answer for it, and a caller reads the false and offers a file
	102	+;; browser instead. An Android jvui would replace them.
	103	+
	104	+(defn pick-image!
	105	+ "False: this platform has no picture chooser."
	106	+ []
	107	+ false)
	108	+
	109	+(defn picked-image!
	110	+ "nil: nothing was ever chosen, because nothing could be."
	111	+ [_path]
	112	+ nil)

++(ns jvui.host
++  "The window as a thing in an operating system, rather than a thing to draw in.
++
++  Its title, the display it is on, the browser, the clipboard, and asking it
++  to close. None of this is painting and none of it belongs in a widget, but
++  a client cannot do any of it either — the window handle lives here, so the
++  questions come here too.
++
++  Everything answers a harmless nothing before a window exists: no title to
++  set, no display to measure, no clipboard to read. A client that runs
++  headless — jvui's own tests do — should not have to guard every call."
++  (:require [jvui.sdl :as sdl]))
++
++(defonce ^:private win (atom nil))
++(defonce ^:private quit? (atom false))
++
++(defn install!
++  "Called by the frame loop when a window opens."
++  [window]
++  (reset! win window)
++  (reset! quit? false))
++
++(defn clear! [] (reset! win nil) nil)
++
++(defn window
++  "The open window, or nil."
++  []
++  @win)
++
++;; --- closing -----------------------------------------------------------------
++
++(defn quit!
++  "Ask the frame loop to stop after the current frame.
++
++  A request rather than an exit: the loop owns teardown — textures, painter,
++  fonts, the window itself — and tearing any of it down from under a walk
++  that is still running is how a close turns into a crash."
++  []
++  (reset! quit? true)
++  nil)
++
++(defn quit-requested? [] @quit?)
++
++;; --- the window --------------------------------------------------------------
++
++(defn set-title! [s]
++  (when-let [w @win] (sdl/set-window-title! w (str s)))
++  nil)
++
++(defn window-size
++  "[w h] in window units, or nil with no window."
++  []
++  (when-let [w @win] (sdl/window-size w)))
++
++(defn window-width
++  "Width in window units, or 0 with no window — a caller dividing by it wants
++  a number, and zero is the honest one."
++  []
++  (or (first (window-size)) 0))
++
++(defn screen-size
++  "[w h] of the display this window is on, or nil with no window."
++  []
++  (when-let [w @win] (sdl/display-bounds w)))
++
++;; --- the world outside -------------------------------------------------------
++
++(defn open-url!
++  "Hand a URL to whatever the desktop opens URLs with."
++  [url]
++  (boolean (and (string? url) (seq url) (sdl/open-url! url))))
++
++(defn clipboard-image-png!
++  "Write the clipboard's picture to `path` as PNG; true when there was one.
++
++  Only image/png is asked for. A clipboard may also hold a bitmap or a TIFF
++  and converting those would mean an encoder in here, which is a lot of
++  toolkit for a paste — and every desktop that puts a picture on a clipboard
++  this decade puts a PNG there too."
++  [path]
++  (boolean
++    (when (sdl/has-clipboard-data "image/png")
++      (when-let [[p n] (sdl/clipboard-data "image/png")]
++        (try
++          (let [bs (byte-array n)]
++            (jolt.ffi/read-array p :byte bs)
++            (java.nio.file.Files/write
++              (java.nio.file.Path/of (str path) (into-array String []))
++              bs
++              (into-array java.nio.file.OpenOption []))
++            true)
++          (finally (sdl/sdl-free! p)))))))
++
++;; --- the picture chooser ------------------------------------------------------
++;; There is not one, and that is the same answer glimmer-vidya gives on a
++;; desktop. The chooser exists so a phone can hand back a grant for one
++;; picture without the app holding a permission over all of them; a desktop
++;; has no such thing and browsing the filesystem is the answer there.
++;;
++;; So these are not stubs waiting to be filled in on this backend — they are
++;; the correct answer for it, and a caller reads the false and offers a file
++;; browser instead. An Android jvui would replace them.
++
++(defn pick-image!
++  "False: this platform has no picture chooser."
++  []
++  false)
++
++(defn picked-image!
++  "nil: nothing was ever chosen, because nothing could be."
++  [_path]
++  nil)

modified jvui/src/jvui/sdl.clj +35 -0 Viewed

@@ -282,6 +282,41 @@
282	282	(ffi/write-array p :int pixels)
283	283	(raw-update-texture tex ffi/null p (* 4 w)))))
284	284
	285	+;; --- the platform around the window -----------------------------------------
	286	+;; Title, display, browser, clipboard. None of it is drawing, and all of it is
	287	+;; what a client asks the toolkit for because the toolkit is the only thing
	288	+;; holding a window handle.
	289	+
	290	+(ffi/defcfn set-window-title! "SDL_SetWindowTitle" [:pointer :string] :bool)
	291	+(ffi/defcfn open-url! "SDL_OpenURL" [:string] :bool)
	292	+(ffi/defcfn display-for-window "SDL_GetDisplayForWindow" [:pointer] :uint)
	293	+(ffi/defcfn ^:private raw-display-bounds "SDL_GetDisplayUsableBounds"
	294	+ [:uint :pointer] :bool)
	295	+(ffi/defcfn has-clipboard-data "SDL_HasClipboardData" [:string] :bool)
	296	+(ffi/defcfn ^:private raw-clipboard-data "SDL_GetClipboardData"
	297	+ [:string :pointer] :pointer)
	298	+(ffi/defcfn sdl-free! "SDL_free" [:pointer] :void)
	299	+
	300	+(defn display-bounds
	301	+ "[w h] of the display `window` is on, minus whatever the desktop reserves
	302	+ for panels — usable bounds rather than raw, because a window sized to the
	303	+ whole display is one with its bottom edge under a taskbar."
	304	+ [window]
	305	+ (ffi/with-alloc [r 16]
	306	+ (when (raw-display-bounds (display-for-window window) r)
	307	+ [(ffi/read (+ r 8) :int) (ffi/read (+ r 12) :int)])))
	308	+
	309	+(defn clipboard-data
	310	+ "The clipboard's contents for `mime`, as [pointer length], or nil.
	311	+
	312	+ The pointer is SDL's and the caller must hand it back to `sdl-free!`."
	313	+ [mime]
	314	+ (ffi/with-alloc [sz 8]
	315	+ (let [p (raw-clipboard-data mime sz)
	316	+ n (ffi/read sz :uint64)]
	317	+ (when (and p (not (ffi/null? p)) (pos? n))
	318	+ [p n]))))
	319	+
285	320	(defn update-texture-raw!
286	321	"Upload `h` rows of `pitch` bytes from FOREIGN memory into the whole of `tex`.
287	322

        (ffi/write-array p :int pixels)
        (raw-update-texture tex ffi/null p (* 4 w)))))
++;; --- the platform around the window -----------------------------------------
++;; Title, display, browser, clipboard. None of it is drawing, and all of it is
++;; what a client asks the toolkit for because the toolkit is the only thing
++;; holding a window handle.
++
++(ffi/defcfn set-window-title! "SDL_SetWindowTitle" [:pointer :string] :bool)
++(ffi/defcfn open-url!         "SDL_OpenURL"        [:string] :bool)
++(ffi/defcfn display-for-window "SDL_GetDisplayForWindow" [:pointer] :uint)
++(ffi/defcfn ^:private raw-display-bounds "SDL_GetDisplayUsableBounds"
++  [:uint :pointer] :bool)
++(ffi/defcfn has-clipboard-data "SDL_HasClipboardData" [:string] :bool)
++(ffi/defcfn ^:private raw-clipboard-data "SDL_GetClipboardData"
++  [:string :pointer] :pointer)
++(ffi/defcfn sdl-free! "SDL_free" [:pointer] :void)
++
++(defn display-bounds
++  "[w h] of the display `window` is on, minus whatever the desktop reserves
++  for panels — usable bounds rather than raw, because a window sized to the
++  whole display is one with its bottom edge under a taskbar."
++  [window]
++  (ffi/with-alloc [r 16]
++    (when (raw-display-bounds (display-for-window window) r)
++      [(ffi/read (+ r 8) :int) (ffi/read (+ r 12) :int)])))
++
++(defn clipboard-data
++  "The clipboard's contents for `mime`, as [pointer length], or nil.
++
++  The pointer is SDL's and the caller must hand it back to `sdl-free!`."
++  [mime]
++  (ffi/with-alloc [sz 8]
++    (let [p (raw-clipboard-data mime sz)
++          n (ffi/read sz :uint64)]
++      (when (and p (not (ffi/null? p)) (pos? n))
++        [p n]))))
++
  (defn update-texture-raw!
    "Upload `h` rows of `pitch` bytes from FOREIGN memory into the whole of `tex`.

added jvui/test/jvui/host_check.clj +52 -0 Viewed

new file mode 100644
@@ -0,0 +1,52 @@
	1	+(ns jvui.host-check
	2	+ "The seven platform calls, against a real window.
	3	+
	4	+ Under SDL's dummy video driver, which gives a window and a renderer and no
	5	+ display. That limits what can be claimed and the claims are limited to
	6	+ match: `open-url!` is not invoked, because succeeding would mean a browser
	7	+ opening on somebody's desktop and failing would only mean this container
	8	+ has none. Everything else is checked for real."
	9	+ (:require [jvui.app :as app]
	10	+ [jvui.host :as host]
	11	+ [jvui.widgets :as w]))
	12	+
	13	+(defn -main [& _]
	14	+ (let [out (atom [])
	15	+ ck! (fn [n ok?] (swap! out conj [n (boolean ok?)]))
	16	+ seen (atom 0)]
	17	+ ;; Before any window: everything must answer harmlessly rather than throw.
	18	+ (ck! "no window: screen-size is nil" (nil? (host/screen-size)))
	19	+ (ck! "no window: window-width is 0" (zero? (host/window-width)))
	20	+ (ck! "no window: set-title! is quiet" (nil? (host/set-title! "x")))
	21	+ (ck! "no window: no clipboard picture" (false? (host/clipboard-image-png! "/tmp/nope.png")))
	22	+ (ck! "no chooser on a desktop" (false? (host/pick-image!)))
	23	+ (ck! "and nothing ever chosen" (nil? (host/picked-image! "/tmp/nope.png")))
	24	+
	25	+ (app/run!
	26	+ (fn []
	27	+ (swap! seen inc)
	28	+ (when (= 1 @seen)
	29	+ (ck! "a window is installed" (some? (host/window)))
	30	+ (ck! "window-width is positive" (pos? (host/window-width)))
	31	+ (let [[sw sh] (or (host/screen-size) [0 0])]
	32	+ (ck! "screen-size answers a display" (and (pos? sw) (pos? sh))))
	33	+ (ck! "set-title! does not throw" (nil? (host/set-title! "retitled")))
	34	+ ;; No picture on this clipboard, so false — the path that matters
	35	+ ;; is that asking is safe, not that a picture appears.
	36	+ (ck! "clipboard with no picture is false"
	37	+ (false? (host/clipboard-image-png! "/tmp/jvui-clip.png"))))
	38	+ ;; Ask to close on the third frame; the loop is allowed six, so
	39	+ ;; stopping earlier is the evidence.
	40	+ (when (= 3 @seen) (host/quit!))
	41	+ (w/page* {} (fn [_ _] (w/label "host"))))
	42	+ {:title "host" :width 200 :height 140 :frames 6})
	43	+
	44	+ (ck! "quit! stopped the loop early" (< @seen 6))
	45	+ (ck! "and the window is released" (nil? (host/window)))
	46	+
	47	+ (doseq [[n ok?] @out] (println (if ok? "- " "FAIL ") n))
	48	+ (let [bad (remove second @out)]
	49	+ (println (if (seq bad)
	50	+ (str (count bad) " of " (count @out) " checks FAILED")
	51	+ (str "all " (count @out) " checks passed")))
	52	+ (when (seq bad) (System/exit 1)))))

++(ns jvui.host-check
++  "The seven platform calls, against a real window.
++
++  Under SDL's dummy video driver, which gives a window and a renderer and no
++  display. That limits what can be claimed and the claims are limited to
++  match: `open-url!` is not invoked, because succeeding would mean a browser
++  opening on somebody's desktop and failing would only mean this container
++  has none. Everything else is checked for real."
++  (:require [jvui.app :as app]
++            [jvui.host :as host]
++            [jvui.widgets :as w]))
++
++(defn -main [& _]
++  (let [out (atom [])
++        ck! (fn [n ok?] (swap! out conj [n (boolean ok?)]))
++        seen (atom 0)]
++    ;; Before any window: everything must answer harmlessly rather than throw.
++    (ck! "no window: screen-size is nil"   (nil? (host/screen-size)))
++    (ck! "no window: window-width is 0"    (zero? (host/window-width)))
++    (ck! "no window: set-title! is quiet"  (nil? (host/set-title! "x")))
++    (ck! "no window: no clipboard picture" (false? (host/clipboard-image-png! "/tmp/nope.png")))
++    (ck! "no chooser on a desktop"         (false? (host/pick-image!)))
++    (ck! "and nothing ever chosen"         (nil? (host/picked-image! "/tmp/nope.png")))
++
++    (app/run!
++      (fn []
++        (swap! seen inc)
++        (when (= 1 @seen)
++          (ck! "a window is installed" (some? (host/window)))
++          (ck! "window-width is positive" (pos? (host/window-width)))
++          (let [[sw sh] (or (host/screen-size) [0 0])]
++            (ck! "screen-size answers a display" (and (pos? sw) (pos? sh))))
++          (ck! "set-title! does not throw" (nil? (host/set-title! "retitled")))
++          ;; No picture on this clipboard, so false — the path that matters
++          ;; is that asking is safe, not that a picture appears.
++          (ck! "clipboard with no picture is false"
++               (false? (host/clipboard-image-png! "/tmp/jvui-clip.png"))))
++        ;; Ask to close on the third frame; the loop is allowed six, so
++        ;; stopping earlier is the evidence.
++        (when (= 3 @seen) (host/quit!))
++        (w/page* {} (fn [_ _] (w/label "host"))))
++      {:title "host" :width 200 :height 140 :frames 6})
++
++    (ck! "quit! stopped the loop early" (< @seen 6))
++    (ck! "and the window is released"   (nil? (host/window)))
++
++    (doseq [[n ok?] @out] (println (if ok? "- " "FAIL ") n))
++    (let [bad (remove second @out)]
++      (println (if (seq bad)
++                 (str (count bad) " of " (count @out) " checks FAILED")
++                 (str "all " (count @out) " checks passed")))
++      (when (seq bad) (System/exit 1)))))