Theseus: a tab can go back into another window

A tab moved to its own window had no way back. Dragging a tab onto
another window's tab bar now moves it there, page and all, and the
right-click menu lists the other windows under Move to window. A
window whose last tab leaves closes.

main.js

@@ -5179,6 +5179,37 @@ function moveTabToNewWindow(id, at) {
   }));
   return true;
 }
+// Moves a tab into an existing window (dest), page and all, at the end of
+// its tab bar, and brings that window forward. A window left without tabs
+// closes, like one whose last tab was closed.
+function moveTabToWindow(id, dest) {
+  const t = tabById(id);
+  if (!t || !t.w || !windows.includes(t.w) || !dest || dest === t.w || !windows.includes(dest)) return false;
+  const src = t.w;
+  withWin(src, () => {
+    if (fsTabId === id) leaveHtmlFullscreen(t, true);
+    const mine = winTabs();
+    const i = mine.indexOf(t);
+    const next = mine[i + 1] || mine[i - 1];
+    try { win.contentView.removeChildView(t.view); } catch {}
+    t.w = null; // between windows
+    if (next) { if (activeId === id) setActive(next.id); else emitTabs(); }
+  });
+  t.w = dest;
+  // After the destination window's last tab in the shared list.
+  tabs.splice(tabs.indexOf(t), 1);
+  const there = winTabs(dest);
+  tabs.splice(there.length ? tabs.indexOf(there[there.length - 1]) + 1 : tabs.length, 0, t);
+  try { dest.win.contentView.addChildView(t.view); } catch {}
+  withWin(dest, () => {
+    if (curW === primaryW && leftPanelKey() && settings.sidebarPanelMode !== "push") raisePanelGrip();
+    setActive(id);
+    layout();
+  });
+  if (!winTabs(src).length) { try { src.win.close(); } catch {} }
+  try { dest.win.show(); dest.win.focus(); t.view.webContents.focus(); } catch {}
+  return true;
+}
 function closeTab(id) {
   const owner = tabById(id)?.w;
   if (owner && owner !== curW) return withWin(owner, () => closeTabHere(id));
@@ -5811,6 +5842,16 @@ ipcMain.handle("tab-detach", (e, tabId, x, y) => {
   const b = win.getBounds();
   const inside = px >= b.x && px < b.x + b.width && py >= b.y && py < b.y + b.height;
   if (inside && py < b.y + CHROME_H + 24) return false;
+  // Let go over another window's toolbar: the tab joins that window. Only
+  // when the point is off its own window, which is the one on top while the
+  // tab is dragged from it.
+  if (!inside) {
+    for (const w of windows) {
+      if (w === curW || !w.win || w.win.isDestroyed() || w.win.isMinimized()) continue;
+      const wb = w.win.getBounds();
+      if (px >= wb.x && px < wb.x + wb.width && py >= wb.y && py < wb.y + (w.chromeH || 84) + 24) return moveTabToWindow(t.id, w);
+    }
+  }
   return moveTabToNewWindow(t.id, { x: px, y: py });
 });
 ipcMain.handle("tab-context-menu-popup", (e, tabId, rect) => {
@@ -5850,6 +5891,12 @@ ipcMain.handle("tab-context-menu-popup", (e, tabId, rect) => {
     { label: "Reload",     click: () => { try { t.view.webContents.reload(); } catch {} } },
     { label: "Duplicate",  click: () => { if (t.url) createTab(t.url, { after: t.id }); else createTab(null, { after: t.id }); } },
     { label: "Move to new window", enabled: winTabs(t.w).length > 1, click: () => moveTabToNewWindow(t.id) },
+    ...(windows.length > 1 ? [{ label: "Move to window", submenu: windows.filter((w) => w !== t.w).map((w) => {
+      const front = tabById(w === curW ? activeId : w.activeId);
+      const n = winTabs(w).length;
+      const name = String(front?.title || front?.url || "Window").slice(0, 60);
+      return { label: n > 1 ? `${name} (and ${n - 1} more)` : name, click: () => moveTabToWindow(t.id, w) };
+    }) }] : []),
     { label: "Group",      submenu: groupSubmenu },
     { label: "Add to Bookmarks", enabled: !bmDisabled, click: () => {
         if (bmDisabled) return;