diff --git a/src/inset.ts b/src/inset.ts
new file mode 100644
index 0000000..d92b823
--- /dev/null
+++ b/src/inset.ts
@@ -0,0 +1,47 @@
+/**
+ * How much of a leaf's bottom is covered by something the operator cannot
+ * move: Obsidian's desktop status bar, a phone's navbar (which paints over a
+ * leaf rather than shortening it), and the software keyboard, which shrinks
+ * the visual viewport while the layout keeps its full height.
+ *
+ * Pure on purpose - the view feeds it live rectangles, the tests feed it the
+ * numbers a phone actually reports.
+ */
+
+export interface Rect {
+  bottom: number;
+  height: number;
+  left: number;
+  right: number;
+  top: number;
+}
+
+export interface Visible {
+  /** `visualViewport.height`: what is left once the keyboard is up. */
+  height: number;
+  /** `visualViewport.offsetTop`: where the visible area starts. */
+  offsetTop: number;
+}
+
+export function insetFor(
+  leaf: Rect,
+  bars: Rect[],
+  visible: Visible | null
+): number {
+  if (leaf.height <= 0) {
+    return 0;
+  }
+  let overlap = 0;
+  for (const bar of bars) {
+    // A bar that does not sit over this leaf horizontally covers nothing of
+    // it - the desktop status bar next to a left-hand sidedock, say.
+    if (bar.height > 0 && bar.left < leaf.right && bar.right > leaf.left) {
+      overlap = Math.max(overlap, leaf.bottom - bar.top);
+    }
+  }
+  if (visible) {
+    const floor = visible.offsetTop + visible.height;
+    overlap = Math.max(overlap, leaf.bottom - floor);
+  }
+  return Math.max(0, Math.round(overlap));
+}
diff --git a/src/view.ts b/src/view.ts
index 445d666..7cefbf7 100644
--- a/src/view.ts
+++ b/src/view.ts
@@ -1,6 +1,8 @@
 import type { ViewStateResult, WorkspaceLeaf } from "obsidian";
 import { ItemView, TFile } from "obsidian";
 import { mount, unmount } from "svelte";
+import type { Rect } from "./inset";
+import { insetFor } from "./inset";
 import type BeaverPlugin from "./main";
 import { PanelState } from "./state.svelte";
 import App from "./ui/app.svelte";
@@ -144,34 +146,26 @@ export class PanelView extends ItemView {
   }
 
   // Whatever floats over the bottom of this leaf, measured rather than
-  // guessed: the desktop status bar, the phone's navbar (which the workspace
-  // does not subtract from a leaf's height), and the software keyboard, which
-  // shrinks the visual viewport without resizing the layout at all. Returns
-  // the inset it wrote, so the settle loop can tell when it stops changing.
+  // guessed. Returns the inset it wrote, so the settle loop can tell when the
+  // reading stops changing; the arithmetic itself lives in ``inset.ts``.
   private inset(): number {
     const box = this.contentEl.getBoundingClientRect();
     if (box.height <= 0) {
       return Number.NaN;
     }
-    let overlap = 0;
-    for (const bar of BOTTOM_BARS) {
-      const node = document.querySelector<HTMLElement>(bar);
-      if (!node || node.offsetParent === null) {
-        continue;
-      }
-      const b = node.getBoundingClientRect();
-      if (b.height > 0 && b.left < box.right && b.right > box.left) {
-        overlap = Math.max(overlap, box.bottom - b.top);
+    const bars: Rect[] = [];
+    for (const selector of BOTTOM_BARS) {
+      const node = document.querySelector<HTMLElement>(selector);
+      if (node && node.offsetParent !== null) {
+        bars.push(node.getBoundingClientRect());
       }
     }
     const vv = window.visualViewport;
-    if (vv) {
-      // The keyboard eats the bottom of the window; anything of this leaf
-      // below that line is unreachable until it goes away.
-      const floor = vv.offsetTop + vv.height;
-      overlap = Math.max(overlap, box.bottom - floor);
-    }
-    const inset = Math.max(0, Math.round(overlap));
+    const inset = insetFor(
+      box,
+      bars,
+      vv ? { height: vv.height, offsetTop: vv.offsetTop } : null
+    );
     this.contentEl.style.setProperty("--beaver-inset-bottom", `${inset}px`);
     return inset;
   }
diff --git a/tests/inset.test.ts b/tests/inset.test.ts
new file mode 100644
index 0000000..939a57b
--- /dev/null
+++ b/tests/inset.test.ts
@@ -0,0 +1,67 @@
+import { describe, expect, it } from "vitest";
+import type { Rect } from "../src/inset";
+import { insetFor } from "../src/inset";
+
+const rect = (over: Partial<Rect> = {}): Rect => ({
+  bottom: 780,
+  height: 780,
+  left: 0,
+  right: 390,
+  top: 0,
+  ...over,
+});
+
+// A phone: 780 tall, a 56px navbar over the bottom, 340px of keyboard.
+const NAVBAR = rect({ bottom: 780, height: 56, top: 724 });
+
+describe("insetFor", () => {
+  it("clears nothing when the leaf stands alone", () => {
+    expect(insetFor(rect(), [], null)).toBe(0);
+  });
+
+  it("clears the navbar that paints over the leaf", () => {
+    expect(insetFor(rect(), [NAVBAR], null)).toBe(56);
+  });
+
+  it("clears the keyboard, which shortens no layout", () => {
+    expect(insetFor(rect(), [], { height: 440, offsetTop: 0 })).toBe(340);
+  });
+
+  it("takes the deeper of the two rather than their sum", () => {
+    // While the keyboard is up the navbar is behind it: adding them would
+    // push the composer twice as far as it has to go.
+    expect(insetFor(rect(), [NAVBAR], { height: 440, offsetTop: 0 })).toBe(340);
+  });
+
+  it("comes back to the navbar once the keyboard goes", () => {
+    expect(insetFor(rect(), [NAVBAR], { height: 780, offsetTop: 0 })).toBe(56);
+  });
+
+  it("ignores a bar that sits beside the leaf, not over it", () => {
+    // The desktop status bar floats bottom-right; a left-hand sidedock never
+    // reaches it.
+    const sidedock = rect({ right: 300 });
+    const statusBar = rect({
+      bottom: 780,
+      height: 20,
+      left: 700,
+      right: 980,
+      top: 760,
+    });
+    expect(insetFor(sidedock, [statusBar], null)).toBe(0);
+  });
+
+  it("measures from where the visible area starts, not from zero", () => {
+    // iOS scrolls the page up under the keyboard instead of resizing it.
+    expect(insetFor(rect(), [], { height: 440, offsetTop: 100 })).toBe(240);
+  });
+
+  it("says nothing about a leaf with no height yet", () => {
+    expect(insetFor(rect({ bottom: 0, height: 0 }), [NAVBAR], null)).toBe(0);
+  });
+
+  it("never returns a negative inset", () => {
+    const short = rect({ bottom: 300, height: 300 });
+    expect(insetFor(short, [], { height: 780, offsetTop: 0 })).toBe(0);
+  });
+});