Fix webview dragging.

src/Pane.zig

@@ -395,6 +395,12 @@ pub fn grabFocus(self: *Pane) void {
     self.content.grabFocus();
 }
 
+/// Whether the content is allowed to be picked as the target of a pointer or a
+/// drop. Off for the length of a pane drag — see `installDropTarget`.
+pub fn setContentTargetable(self: *Pane, targetable: bool) void {
+    self.content.widget().setCanTarget(@intFromBool(targetable));
+}
+
 /// The terminal this pane holds, or null if it holds something else. Callers
 /// that only make sense for a terminal — pasting a VT sequence, say — use this
 /// to opt out on a web pane.
@@ -585,6 +591,17 @@ fn installDropTarget(self: *Pane) void {
     _ = gtk.DropTarget.signals.motion.connect(target, *Pane, &onDropMotion, self, .{});
     _ = gtk.DropTarget.signals.leave.connect(target, *Pane, &onDropLeave, self, .{});
     _ = gtk.DropTarget.signals.drop.connect(target, *Pane, &onDrop, self, .{});
+
+    // On the pane rather than the header, so the whole pane is a target and
+    // `targetAt` has an area to pick a side from. That puts it above the
+    // content in the widget tree, which is fine for a terminal — it takes no
+    // drops — but a web view is a drop target in its own right and an
+    // innermost one wins. Since the preview slides the dragged pane in under
+    // the pointer, the drop at the end of a drag of a browser or a review pane
+    // would land in that pane's own page: no `drop` here, so `endDrag` would
+    // find the drag uncommitted and undo the whole move. `View.beginDrag`
+    // takes every pane's content out of the picking for the length of a drag
+    // so that the pane is what the drop reaches.
     self.widget().addController(target.as(gtk.EventController));
 }
 

src/View.zig

@@ -786,6 +786,13 @@ pub fn beginDrag(self: *View, pane: *Pane) bool {
         .origin_ratio = position.ratio,
     };
     pane.widget().addCssClass("dragging");
+
+    // For the length of the drag, a pane's content is not a drop target and
+    // the pane itself is. Without this a web view — a browser or a review
+    // pane — swallows the drop that lands on it, including the drop on the
+    // dragged pane itself, which is where the preview has just put it. See
+    // `Pane.installDropTarget`.
+    for (self.panes.items) |p| p.setContentTargetable(false);
     return true;
 }
 
@@ -808,6 +815,10 @@ pub fn commitDrag(self: *View) void {
 
 /// End of a drag. If no drop was accepted, undo whatever the preview did.
 pub fn endDrag(self: *View) void {
+    // Before the early return: a drag whose pane was closed under it has
+    // already cleared `drag`, and the content would stay unpickable for good.
+    for (self.panes.items) |p| p.setContentTargetable(true);
+
     const drag = self.drag orelse return;
     self.drag = null;
     drag.pane.widget().removeCssClass("dragging");