fix: let a Pro tab move or split when dropped on a composer or calendar pane

components/calendar/calendar-month-view.tsx

@@ -328,10 +328,11 @@ export function CalendarMonthView({
   }, []);
 
   const handleCellDrop = useCallback(async (e: DragEvent<HTMLDivElement>, day: Date) => {
-    e.preventDefault();
-    setDropDayKey(null);
+    // Anything else (a Pro tab or message moving to this pane) is not ours.
     const json = e.dataTransfer.getData("application/x-calendar-event");
     if (!json) return;
+    e.preventDefault();
+    setDropDayKey(null);
     try {
       const data = JSON.parse(json);
       const originalStart = parseISO(data.originalStart);

components/email/email-composer.tsx

@@ -79,6 +79,7 @@ import { useProMultiAccountIdentities, stripCrossAccountIdentityPrefix } from "@
 import { useAccountStore } from "@/stores/account-store";
 import { useSettingsStore } from "@/stores/settings-store";
 import { ComposerPanelToggle, ComposerPluginSidebar, useComposerSidebars } from "./composer-plugin-sidebars";
+import { dragKindFromTypes } from "@/components/pro/pro-shell-drop";
 import { ComposerSelectionMenu, useComposerSelectionMenu } from "./composer-selection-menu";
 import { PluginSlot } from "@/components/plugins/plugin-slot";
 import { usePluginSlotOffers } from "@/hooks/use-plugin-slot-offers";
@@ -168,6 +169,10 @@ import { usePolicyStore } from "@/stores/policy-store";
 import { registerComposeFlush } from "@/lib/compose-flush-registry";
 import { toUnicodeEmail } from "@/lib/idn";
 
+// A Pro tab, message or folder dragged across a composer tab is on its way to
+// the pane or the split edge: those drags belong to the shell.
+const isShellDrag = (e: React.DragEvent) => dragKindFromTypes(e.dataTransfer.types) !== null;
+
 /**
  * Derives the text/plain alternative from the composer's HTML body, preserving
  * line structure from block elements and <br> tags. Paragraph spacing is on so
@@ -2484,6 +2489,7 @@ export function EmailComposer({
   }, [clearDragState]);
 
   const handleDragEnter = useCallback((e: React.DragEvent) => {
+    if (isShellDrag(e)) return;
     e.preventDefault();
     e.stopPropagation();
     if (e.dataTransfer.types.includes('Files')) {
@@ -2493,18 +2499,21 @@ export function EmailComposer({
   }, [resetDragTimeout]);
 
   const handleDragLeave = useCallback((e: React.DragEvent) => {
+    if (isShellDrag(e)) return;
     e.preventDefault();
     e.stopPropagation();
     resetDragTimeout();
   }, [resetDragTimeout]);
 
   const handleDragOver = useCallback((e: React.DragEvent) => {
+    if (isShellDrag(e)) return;
     e.preventDefault();
     e.stopPropagation();
     resetDragTimeout();
   }, [resetDragTimeout]);
 
   const handleDrop = useCallback((e: React.DragEvent) => {
+    if (isShellDrag(e)) return;
     e.preventDefault();
     e.stopPropagation();
     clearDragState();
@@ -4670,6 +4679,7 @@ export function EmailComposer({
                 }}
                 onDragLeave={() => setIsDraggingChipOverCc(false)}
                 onDrop={(e) => {
+                  if (isShellDrag(e)) return;
                   e.preventDefault();
                   setIsDraggingChipOverCc(false);
                   const raw = e.dataTransfer.getData('application/x-recipient-chip');
@@ -4694,6 +4704,7 @@ export function EmailComposer({
                 }}
                 onDragLeave={() => setIsDraggingChipOverBcc(false)}
                 onDrop={(e) => {
+                  if (isShellDrag(e)) return;
                   e.preventDefault();
                   setIsDraggingChipOverBcc(false);
                   const raw = e.dataTransfer.getData('application/x-recipient-chip');
@@ -6023,6 +6034,7 @@ export function RecipientChipInput({
   // Insert the dragged chip at `target`. Same-field is a local reorder;
   // cross-field routes through onMoveChip with the destination index (#593).
   const performDrop = (e: React.DragEvent, target: number) => {
+    if (isShellDrag(e)) return;
     e.preventDefault();
     setIsDragOver(false);
     setDropIndex(null);

hooks/__tests__/use-time-grid-interactions.test.ts

@@ -215,6 +215,21 @@ describe.each<["0.16" | "1.0"]>([["0.16"], ["1.0"]])("useTimeGridInteractions mo
 // The grid places events by wall clock (7845b2ca). On 2026-10-25 Berlin's
 // 02:00-03:00 happens twice, so a resize across it has to land where the
 // edge was dropped, not an hour off (#1348).
+describe("useTimeGridInteractions drops that are not events", () => {
+  it("leaves a Pro tab dropped on a column to the shell", async () => {
+    const { result } = renderHook(() => useTimeGridInteractions({
+      hourHeight: 60,
+      onCreateRange: () => {},
+      errorMessages,
+    } as never));
+    const preventDefault = vi.fn();
+    const e = { preventDefault, dataTransfer: { getData: () => "" } } as unknown as DragEvent<HTMLDivElement>;
+    await act(() => result.current.handleColumnDrop(e, new Date(2026, 8, 3)));
+    // The shell's own drop handler stands down on a prevented event.
+    expect(preventDefault).not.toHaveBeenCalled();
+  });
+});
+
 describe("useTimeGridInteractions resize on a DST change day (#1348)", () => {
   const original = useCalendarStore.getState().updateEvent;
   const originalTimeZone = useSettingsStore.getState().timeZone;

hooks/use-time-grid-interactions.ts

@@ -398,10 +398,11 @@ export function useTimeGridInteractions({
   }, []);
 
   const handleColumnDrop = useCallback(async (e: DragEvent<HTMLDivElement>, day: Date) => {
-    e.preventDefault();
-    setDropTarget(null);
+    // Anything else (a Pro tab or message moving to this pane) is not ours.
     const json = e.dataTransfer.getData("application/x-calendar-event");
     if (!json) return;
+    e.preventDefault();
+    setDropTarget(null);
     try {
       const data = JSON.parse(json);
       const minutes = snapDragMinutes(e);