diff --git a/docs/superpowers/specs/2026-07-15-auto-hide-caption-design.md b/docs/superpowers/specs/2026-07-15-auto-hide-caption-design.md
new file mode 100644
index 0000000..7d9499e
--- /dev/null
+++ b/docs/superpowers/specs/2026-07-15-auto-hide-caption-design.md
@@ -0,0 +1,90 @@
+# Auto-Hide Caption (frame visible only when active) — Design
+
+**Date:** 2026-07-15
+**Status:** Approved (brainstorming)
+
+## Goal
+
+Show the calculator window's title bar and close button **only while the window is
+active**. When focus moves to another window the calculator stays on top as a clean,
+chromeless display (no caption, no X); clicking it back (or showing it from the tray)
+brings the frame back.
+
+## Behavior
+
+- Handle `WM_ACTIVATE` in `CCalculatorWindow`:
+  - **Activated** (`WA_ACTIVE` / `WA_CLICKACTIVE`): add `WS_CAPTION | WS_SYSMENU`.
+  - **Deactivated** (`WA_INACTIVE`): remove `WS_CAPTION | WS_SYSMENU`.
+- The toggle uses `SetWindowLongPtr(GWL_STYLE)` followed by
+  `SetWindowPos(SWP_FRAMECHANGED | SWP_NOACTIVATE | SWP_NOZORDER)`.
+- `CreateStandalone` is unchanged (window is created with the caption styles; it is
+  hidden at that point, so the first activation just confirms the state).
+- A private helper `ApplyFrameStyle(bool visible)` centralizes the style/geometry change;
+  a `bool m_captionVisible` member tracks the current state and makes the call idempotent
+  (no-op when the requested state already applies).
+
+## Geometry — display stays put
+
+When the frame toggles, the **client rectangle keeps its exact screen position**: the
+caption grows upward when it appears and retracts when it disappears; the displayed
+number never moves.
+
+- On toggle: take the current client rect in screen coordinates
+  (`GetClientRect` + `ClientToScreen`), compute the new window rect for the new style
+  with `AdjustWindowRectEx`, and pass position + size to the same `SetWindowPos` call
+  that applies `SWP_FRAMECHANGED`.
+- Safety clamp: if growing upward would push the window top above the work area top,
+  clamp to the work area (in practice the window sits near the taskbar, so this is a
+  guard, not an expected path).
+
+## Position persistence — canonical form
+
+`OnExitSizeMove` currently saves the window's top-left corner, which now depends on
+whether the caption is present (± caption height). To keep the restored position stable
+across restarts regardless of the frame state at save time:
+
+- **Save** the position in canonical form: the top-left of the **chromeless** window
+  (equal to the client origin, since a plain `WS_POPUP` window has no frame offset).
+- **Restore** (`PlaceInitial`) converts the canonical point back to the window origin for
+  the style in effect at that moment (via `AdjustWindowRectEx`): since the window is
+  created with the caption styles, its top-left is placed one caption height above the
+  saved client origin, so the client lands exactly where it was saved.
+- Registry value name and type are unchanged (`REG_DWORD` pair via `Settings`); existing
+  saved positions are reinterpreted with at most a caption-height offset once, then
+  stay stable.
+
+## Accepted side effects
+
+- Opening the tray menu or the About dialog deactivates the calculator, so the caption
+  disappears while they are open. Consistent with the rule ("active = framed"); no
+  functional impact.
+- The frame appears/disappears instantly (no animation); with DWM on Windows 11 the
+  repaint is near-imperceptible.
+
+## Components touched
+
+`CalculatorWindow.h` / `CalculatorWindow.cpp` only:
+
+- `MESSAGE_HANDLER(WM_ACTIVATE, OnActivate)` + `OnActivate` implementation.
+- Private `void ApplyFrameStyle(bool visible)` + `bool m_captionVisible`.
+- Canonical-position adjustment in `OnExitSizeMove` and `PlaceInitial`.
+
+Engine (`calc`), `Settings` storage format, `App.cpp`, tray icon, and packaging are all
+untouched.
+
+## Testing
+
+- **Engine regression:** `tests\build-and-run.cmd` still passes 16/16 (feature is UI-only).
+- **Manual verification checklist:**
+  1. Show from tray → window appears with caption + X.
+  2. Click another app → caption disappears; the number display does not move.
+  3. Click the calculator → caption reappears; display does not move.
+  4. X → hides to tray (existing `WM_CLOSE` behavior).
+  5. Drag by caption and by body both work; position persists.
+  6. Restart the app → window reappears at the same visual position (no drift).
+
+## Out of scope (YAGNI)
+
+- Animated caption transitions.
+- Custom-drawn caption (approach B — fallback if the native toggle disappoints).
+- Any change to sizing, fonts, or engine behavior.