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/platform-notes.md

@@ -8,7 +8,9 @@ macOS is the primary target; Windows 11+ x64 and Linux x64 are **beta** targets
 
 | Concern | File | mac | win | linux |
 |---|---|---|---|---|
-| Window chrome | `src/bun/windows.ts`, `index.ts` | hiddenInset + traffic-light offset, UI drag header | native titlebar, compact header | same as win |
+| Window chrome (modern skin) | `src/bun/windows.ts`, `index.ts` | hiddenInset + traffic-light offset, UI drag header | native titlebar, compact header | same as win |
+| Window chrome (win7 skin) | `src/bun/windows.ts chromeOptions` | `hidden` (clears Titled → no traffic lights) | frameless | `gtk_window_set_decorated(false)` |
+| Window **resize** | `ui/.../7like/window-resize/` | native frame resizes; win7 frameless uses the synthesized handles | as mac | as mac |
 | External editor launch | `src/bun/edits.ts launchEditor` | `open [-a app]` | `cmd /c start "" [app] file` | app directly / `xdg-open` |
 | Editor app picker | `rpc-handlers.ts pickEditorApp` | /Applications, `.app` | %ProgramFiles%, `.exe` | home, any |
 | Owner/group names | `fs/local.ts resolveId` | `id -nu` + `dscacheutil` | undefined → "—" | `id -nu` + `getent` |
@@ -31,6 +33,27 @@ the default app, copy path) cross to bun, via the `shellItemAction` RPC.
 Remote (SFTP) paths are ALWAYS POSIX regardless of host or server OS
 (`path.posix` throughout `fs/sftp.ts`).
 
+### Frameless windows lose the WM's resize borders (win7 skin, every platform)
+
+`titleBarStyle: "hidden"` is what lets 7.css draw the Aero caption — on Linux the
+GTK wrapper honours it with `gtk_window_set_decorated(false)`. Undecorating also
+removes the window manager's resize borders and corner grips, and electrobun
+1.18.1 offers **no** native resize gesture to put back: the shipped
+`libNativeWrapper` exports `startWindowMove`/`stopWindowMove` with no resize
+counterpart, does not link `gtk_window_begin_resize_drag`, and `BrowserWindow`
+has neither `setResizable` nor min-size constructor options. The window is still
+*programmatically* resizable (`setFrame`/`getFrame` work, and
+`gtk_window_set_resizable(false)` is never called), so the app **synthesizes the
+input affordance**: eight handles in the webview drag the native frame over RPC
+(`ui/src/app/skins/7like/window-resize/`).
+
+That is a **stopgap** — each drag tick is an RPC hop plus an FFI call, where a
+real WM grab is compositor-side. `scripts/check-electrobun-resize.ts` watches the
+installed electrobun for the missing primitives and prints the removal checklist
+when they appear; it runs from `postinstall`, so an electrobun bump surfaces it
+without anyone remembering to look. The modern skin keeps the native frame and
+needs none of this.
+
 ## Memory footprint (expected, not a bug)
 
 Investigated 2026-07 — packaged build. Roughly **~200 MB macOS / ~300 MB