fix(ui,preview): panels chase the keyboard by frame, and the preview is finally mobile

The panel measured itself once, a fixed delay into the keyboard's animation,
so it landed against a viewport that was still moving - and in a sidedock
drawer no resize event fires at all, so it never moved. It now re-places every
frame until its own position holds still, which needs neither a duration to
guess nor an event to wait for. Its input taking focus is another trigger:
that is what opens the keyboard in the first place.

The preview had no viewport meta, so a 390px window still laid out at 980px:
every "mobile" screenshot was a desktop one at a phone's aspect, and
coarse-pointer rules could not apply. Which is why the tap targets looked
fine here and were not.

preview/index.html

@@ -2,6 +2,9 @@
 <html lang="en">
   <head>
     <meta charset="utf-8" />
+    <!-- Without this a phone-sized window still lays out at 980px, so a
+         "mobile" screenshot is not one and coarse-pointer rules never apply. -->
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
     <title>beaver-calendar preview</title>
     <link rel="stylesheet" href="/app.css" />
     <link rel="stylesheet" href="/theme.css" />

src/ui/anchored-panel.svelte

@@ -27,8 +27,9 @@
 
   const view = useView();
 
-  // iOS animates the keyboard in; the viewport only tells the truth after.
+  // The keyboard animates both ways: rather than guess how long that takes,
-  const KEYBOARD_SETTLE_MS = 350;
+  // the panel re-places every frame until its position holds still.
+  const STEADY_FRAMES = 4;
 
   let panel = $state<HTMLElement | null>(null);
   let at = $state({ top: 0, left: 0 });
@@ -74,14 +75,38 @@
       if (!node.contains(event.target as Node)) onclose();
     };
     const reflow = (): void => measure();
+    /**
+     * Re-place every frame until the position holds still. The keyboard
+     * animates both ways, and in a sidedock nothing fires an event at all -
+     * a frame loop needs neither a duration to guess nor an event to wait for.
+     */
+    let frame: number | null = null;
+    const settle = (): void => {
+      if (frame !== null) return;
+      let last = Number.NaN;
+      let steady = 0;
+      const tick = (): void => {
+        measure();
+        steady = at.top === last ? steady + 1 : 0;
+        last = at.top;
+        if (steady >= STEADY_FRAMES) {
+          frame = null;
+          return;
+        }
+        frame = win.requestAnimationFrame(tick);
+      };
+      frame = win.requestAnimationFrame(tick);
+    };
     document.addEventListener("pointerdown", outside, true);
     window.addEventListener("resize", reflow);
-    // The keyboard changes the visual viewport without resizing the window,
+    // The keyboard changes the visual viewport without resizing the window.
-    // and its animation takes a moment to settle.
     const vv = win.visualViewport;
-    vv?.addEventListener("resize", reflow);
+    vv?.addEventListener("resize", settle);
-    vv?.addEventListener("scroll", reflow);
+    vv?.addEventListener("scroll", settle);
-    const settle = win.setTimeout(reflow, KEYBOARD_SETTLE_MS);
+    // The panel's own input takes focus a beat before the keyboard is up, and
+    // swapping fields re-opens it.
+    node.addEventListener("focusin", settle);
+    settle();
     // A panel whose body grew (an option list opened) re-places itself. Only
     // the content height is watched: measure() sets max-height, so observing
     // the panel itself would feed its own change back in.
@@ -97,9 +122,10 @@
       destroy() {
         document.removeEventListener("pointerdown", outside, true);
         window.removeEventListener("resize", reflow);
-        vv?.removeEventListener("resize", reflow);
+        vv?.removeEventListener("resize", settle);
-        vv?.removeEventListener("scroll", reflow);
+        vv?.removeEventListener("scroll", settle);
-        win.clearTimeout(settle);
+        node.removeEventListener("focusin", settle);
+        if (frame !== null) win.cancelAnimationFrame(frame);
         watcher?.disconnect();
         win.removeEventListener("keydown", keys, true);
         panel = null;