cursor: implement implicit grabs

When a button is held down and the cursor leaves a surface, events now
continue to be sent to the client. This allows e.g. dragging a scroll
bar from outside the surface.

river/Cursor.zig

@@ -42,18 +42,21 @@ const ResizeData = struct {
 
 const Mode = union(enum) {
     passthrough: void,
+    down: *View,
     move: *View,
     resize: ResizeData,
 
     /// Enter move or resize mode
     fn enter(self: *Self, mode: @TagType(Mode), event: *c.wlr_event_pointer_button, view: *View) void {
-        std.debug.assert(self.mode == .passthrough);
-
         log.debug(.cursor, "enter {} mode", .{@tagName(mode)});
 
+        switch (mode) {
+            .passthrough => unreachable,
+            .down => self.mode = .{ .down = view },
+            .move, .resize => {
                 const cur_box = &view.current.box;
                 self.mode = switch (mode) {
-            .passthrough => unreachable,
+                    .passthrough, .down => unreachable,
                     .move => .{ .move = view },
                     .resize => .{
                         .resize = .{
@@ -80,14 +83,25 @@ const Mode = union(enum) {
                     if (mode == .move) "move" else "se-resize",
                     self.wlr_cursor,
                 );
+            },
+        }
     }
 
-    /// Return from move/resize to passthrough
+    /// Return from down/move/resize to passthrough
     fn leave(self: *Self, event: *c.wlr_event_pointer_button) void {
         std.debug.assert(self.mode != .passthrough);
 
         log.debug(.cursor, "leave {} mode", .{@tagName(self.mode)});
 
+        // If we were in down mode, we need pass along the release event
+        if (self.mode == .down)
+            _ = c.wlr_seat_pointer_notify_button(
+                self.seat.wlr_seat,
+                event.time_msec,
+                event.button,
+                event.state,
+            );
+
         self.mode = .passthrough;
         passthrough(self, event.time_msec);
     }
@@ -100,6 +114,15 @@ const Mode = union(enum) {
                 c.wlr_cursor_move(self.wlr_cursor, device, delta_x, delta_y);
                 passthrough(self, time);
             },
+            .down => |view| {
+                c.wlr_cursor_move(self.wlr_cursor, device, delta_x, delta_y);
+                c.wlr_seat_pointer_notify_motion(
+                    self.seat.wlr_seat,
+                    time,
+                    self.wlr_cursor.x - @intToFloat(f64, view.current.box.x),
+                    self.wlr_cursor.y - @intToFloat(f64, view.current.box.y),
+                );
+            },
             .move => |view| {
                 var output_width: c_int = undefined;
                 var output_height: c_int = undefined;
@@ -364,6 +387,8 @@ fn handleButton(listener: ?*c.wl_listener, data: ?*c_void) callconv(.C) void {
                         else => {},
                     }
                     return;
+                } else {
+                    Mode.enter(self, .down, event, view);
                 }
             }
         }

river/InputManager.zig

@@ -108,6 +108,7 @@ pub fn isCursorActionTarget(self: Self, view: *View) bool {
         const seat = &node.data;
         switch (seat.cursor.mode) {
             .passthrough => {},
+            .down => |target_view| if (target_view == view) break true,
             .move => |target_view| if (target_view == view) break true,
             .resize => |data| if (data.view == view) break true,
         }