Synthesize window resize for the frameless win7 skin (a documented stopgap)

The win7 skin is frameless so 7.css can draw the Aero caption, and on GTK that
means undecorated, which takes the window manager's resize borders and corner
grips with it. All three win7 windows were completely unresizable.

electrobun 1.18.1 offers nothing to re-enable, verified against the shipped
binary rather than assumed: libNativeWrapper exports startWindowMove/
stopWindowMove with no resize counterpart, imports gtk_window_begin_move_drag
but not gtk_window_begin_resize_drag, dragRegions.ts implements moving only,
and BrowserWindow has neither setResizable nor min-size options. But the
window is still programmatically resizable, so only the input affordance is
missing: eight handles overlay the frame and drag it through a new
windowGetFrame/windowSetFrame pair on the shared windowControlHandlers factory,
which gives all three window kinds resize with no per-window wiring.

Three things that would have made this quietly wrong:

- dragRegions.ts starts a window MOVE from a mousedown listener on document,
  never checks defaultPrevented, and is not suppressed by a pointerdown
  handler either (the compatibility mousedown still fires). Every handle
  carries app-region-no-drag, which short-circuits its check first.
- deltas come from screenX, not clientX: a north/west resize moves the window
  origin under a stationary pointer, so client coords feed that back into the
  next delta and the window runs away.
- the min-size clamp re-anchors the opposite edge; clamping width alone lets x
  keep tracking the pointer, walking the window off-screen while it looks
  frozen.

This is a stopgap and the point is that it is removable on sight. The frame
math is a dependency-free module so scripts/resize-math-probe.ts asserts all
eight edges without a window (verified it catches the unanchored-clamp bug:
x jumps to 5100 instead of parking at 460). scripts/check-electrobun-resize.ts
scans the installed electrobun for the primitives this is waiting on and prints
the removal checklist when they appear; it runs from postinstall so a version
bump surfaces it, never fails an install, and is silent otherwise. Both trigger
paths were simulated. The component header carries the full checklist; docs
cover the platform consequence and the dragRegions traps.

Harness diffs exactly 0 in both themes; hover and menu probes still clean.
Not yet exercised with a real pointer — that needs a live window.

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

docs/gotchas.md

@@ -266,6 +266,31 @@ touching the adjacent code.
   pattern appears in the killing command's OWN argv. Break the literal with a
   character class — `pkill -f 'dev-linux-x[6]4'` — the way `ps | grep [x]yz`
   has always worked.
+- **A frameless window cannot be resized, and `preventDefault` will not save
+  you from the drag region.** Undecorating the GTK window (what
+  `titleBarStyle: "hidden"` does) takes the WM's resize borders with it, and
+  electrobun exposes no resize gesture — hence the synthesized handles in
+  `ui/src/app/skins/7like/window-resize/` (a STOPGAP; that file carries the
+  removal checklist, and `scripts/check-electrobun-resize.ts` fires from
+  `postinstall` when upstream makes it deletable). Three traps live in that
+  code, all verified against `dist/api/bun/preload/dragRegions.ts`:
+  - It starts a window **move** from a `mousedown` listener on `document`, in
+    the **bubble** phase. It never reads `defaultPrevented`, so
+    `event.preventDefault()` does **nothing** to stop it.
+  - A `pointerdown` handler does not stop it either: the compatibility
+    `mousedown` still fires afterwards, and stopping propagation on a *pointer*
+    event has no effect on a *mouse* event. This is the trap that looks fixed
+    in every test until the window jumps.
+  - `isAppRegionDrag` checks `.electrobun-webkit-app-region-no-drag` **first**
+    and short-circuits, so that class — on the handle itself — is the only
+    reliable suppressor. Without it, every resize drag also drags the window.
+  Also: deltas must come from `screenX/screenY`. North/west resizes move the
+  window's origin, which moves the webview's client origin under a stationary
+  pointer, so `clientX` feeds that movement back into the next delta and the
+  window runs away. And the minimum-size clamp must re-anchor the opposite
+  edge — clamping the width alone lets `x` keep tracking the pointer, walking
+  the window off-screen while it looks frozen (`scripts/resize-math-probe.ts`
+  asserts exactly that, and was verified to catch it).
 - **Prefer the headless probes over launching the app.** `bun run start` opens a
   real window that takes focus, which interrupts whatever the user is doing;
   there is no Xvfb/xvfb-run on this machine and Wayland offers no