feat(electrobun): native GTK drag/resize via gtk_window_begin_resize_drag

- gtk-window.ts: FFI wrapper calling libgtk-3.so.0 directly via bun:ffi
- begin_resize_drag: delegates resize to window manager (zero CPU, smooth)
- begin_move_drag: delegates move to window manager (replaces JS drag)
- Removed all JavaScript-based drag/resize logic (no mousemove/mouseup)
- RPC: window.beginResize + window.beginMove
- Resize handles: 4px edges + 8px corners with proper cursors

ui-electrobun/src/bun/gtk-window.ts

@@ -0,0 +1,105 @@
+/**
+ * GTK3 FFI — direct calls to libgtk-3.so.0 for window management.
+ *
+ * Used for begin_resize_drag and begin_move_drag which Electrobun
+ * doesn't expose natively. These delegate to the window manager
+ * for smooth, zero-CPU resize/move behavior.
+ */
+
+import { dlopen, FFIType, ptr } from "bun:ffi";
+
+// GdkWindowEdge values
+export const GDK_EDGE = {
+  NW: 0, N: 1, NE: 2,
+  W: 3,        E: 4,
+  SW: 5, S: 6, SE: 7,
+} as const;
+
+type GdkEdge = (typeof GDK_EDGE)[keyof typeof GDK_EDGE];
+
+let gtk3: ReturnType<typeof dlopen> | null = null;
+
+function getGtk() {
+  if (gtk3) return gtk3;
+  try {
+    gtk3 = dlopen("libgtk-3.so.0", {
+      gtk_window_begin_resize_drag: {
+        args: [FFIType.ptr, FFIType.i32, FFIType.i32, FFIType.i32, FFIType.i32, FFIType.u32],
+        returns: FFIType.void,
+      },
+      gtk_window_begin_move_drag: {
+        args: [FFIType.ptr, FFIType.i32, FFIType.i32, FFIType.i32, FFIType.u32],
+        returns: FFIType.void,
+      },
+    });
+    return gtk3;
+  } catch (err) {
+    console.error("[gtk-window] Failed to dlopen libgtk-3.so.0:", err);
+    return null;
+  }
+}
+
+/**
+ * Delegate resize to the window manager.
+ * The WM handles cursor, animation, constraints — zero CPU from us.
+ */
+export function beginResizeDrag(
+  windowPtr: number | bigint,
+  edge: GdkEdge,
+  button: number,
+  rootX: number,
+  rootY: number,
+) {
+  const lib = getGtk();
+  if (!lib) return false;
+  try {
+    lib.symbols.gtk_window_begin_resize_drag(
+      windowPtr as any,
+      edge,
+      button,
+      Math.round(rootX),
+      Math.round(rootY),
+      0, // GDK_CURRENT_TIME
+    );
+    return true;
+  } catch (err) {
+    console.error("[gtk-window] begin_resize_drag failed:", err);
+    return false;
+  }
+}
+
+/**
+ * Delegate move to the window manager.
+ */
+export function beginMoveDrag(
+  windowPtr: number | bigint,
+  button: number,
+  rootX: number,
+  rootY: number,
+) {
+  const lib = getGtk();
+  if (!lib) return false;
+  try {
+    lib.symbols.gtk_window_begin_move_drag(
+      windowPtr as any,
+      button,
+      Math.round(rootX),
+      Math.round(rootY),
+      0,
+    );
+    return true;
+  } catch (err) {
+    console.error("[gtk-window] begin_move_drag failed:", err);
+    return false;
+  }
+}
+
+// Edge string → GDK_EDGE mapping
+const EDGE_MAP: Record<string, GdkEdge> = {
+  n: GDK_EDGE.N, s: GDK_EDGE.S, e: GDK_EDGE.E, w: GDK_EDGE.W,
+  ne: GDK_EDGE.NE, nw: GDK_EDGE.NW, se: GDK_EDGE.SE, sw: GDK_EDGE.SW,
+};
+
+export function edgeStringToGdk(edge: string): GdkEdge | null {
+  return EDGE_MAP[edge] ?? null;
+}

ui-electrobun/src/bun/index.ts

@@ -108,6 +108,24 @@ const rpc = BrowserView.defineRPC<PtyRPCSchema>({
       ...providerHandlers,
       ...miscHandlers,
 
+      // GTK native drag/resize — delegates to window manager (zero CPU)
+      "window.beginResize": ({ edge, button, rootX, rootY }: { edge: string; button: number; rootX: number; rootY: number }) => {
+        try {
+          const { beginResizeDrag, edgeStringToGdk } = require("./gtk-window.ts");
+          const gdkEdge = edgeStringToGdk(edge);
+          if (gdkEdge === null) return { ok: false, error: `Unknown edge: ${edge}` };
+          const ok = beginResizeDrag((mainWindow as any).ptr, gdkEdge, button, rootX, rootY);
+          return { ok };
+        } catch (err) { console.error("[window.beginResize]", err); return { ok: false }; }
+      },
+      "window.beginMove": ({ button, rootX, rootY }: { button: number; rootX: number; rootY: number }) => {
+        try {
+          const { beginMoveDrag } = require("./gtk-window.ts");
+          const ok = beginMoveDrag((mainWindow as any).ptr, button, rootX, rootY);
+          return { ok };
+        } catch (err) { console.error("[window.beginMove]", err); return { ok: false }; }
+      },
+
       // Window controls — need mainWindow closure, stay inline
       "window.minimize": () => { try { mainWindow.minimize(); return { ok: true }; } catch (err) { console.error("[window.minimize]", err); return { ok: false }; } },
       "window.maximize": () => {