WSWindowManager: Ensure that we pick a single window to deliver a full stream of events to

This is effectively a mouse grab except that we don't require any client
coordination to request it (which is probably OK, and certainly a lot
simpler to implement).

This prevents e.g. dragging the mouse cursor out of paint and over the
terminal from selecting text unexpectedly.

Servers/WindowServer/WSWindowManager.cpp

@@ -703,6 +703,19 @@ void WSWindowManager::process_mouse_event(WSMouseEvent& event, WSWindow*& hovere
 
     WSWindow* event_window_with_frame = nullptr;
 
+    if (m_active_input_window) {
+        // At this point, we have delivered the start of an input sequence to a
+        // client application. We must keep delivering to that client
+        // application until the input sequence is done.
+        //
+        // This prevents e.g. dragging on one window out of the bounds starting
+        // a drag in that other unrelated window, and other silly shennanigans.
+        auto translated_event = event.translated(-m_active_input_window->position());
+        deliver_mouse_event(*m_active_input_window, translated_event);
+        if (event.type() == WSEvent::MouseUp && event.buttons() == 0) {
+            m_active_input_window = nullptr;
+        }
+    } else {
         for_each_visible_window_from_front_to_back([&](WSWindow& window) {
             auto window_frame_rect = window.frame().rect();
             if (!window_frame_rect.contains(event.position()))
@@ -724,6 +737,8 @@ void WSWindowManager::process_mouse_event(WSMouseEvent& event, WSWindow*& hovere
                     start_window_resize(window, event);
                     return IterationDecision::Break;
                 }
+            }
+
             if (m_keyboard_modifiers == Mod_Logo && event.type() == WSEvent::MouseWheel) {
                 float opacity_change = -event.wheel_delta() * 0.05f;
                 float new_opacity = window.opacity() + opacity_change;
@@ -735,7 +750,7 @@ void WSWindowManager::process_mouse_event(WSMouseEvent& event, WSWindow*& hovere
                 window.invalidate();
                 return IterationDecision::Break;
             }
-        }
+
             // Well okay, let's see if we're hitting the frame or the window inside the frame.
             if (window.rect().contains(event.position())) {
                 if (window.type() == WSWindowType::Normal && event.type() == WSEvent::MouseDown)
@@ -745,6 +760,9 @@ void WSWindowManager::process_mouse_event(WSMouseEvent& event, WSWindow*& hovere
                 if (!window.global_cursor_tracking() && !windows_who_received_mouse_event_due_to_cursor_tracking.contains(&window)) {
                     auto translated_event = event.translated(-window.position());
                     deliver_mouse_event(window, translated_event);
+                    if (event.type() == WSEvent::MouseDown) {
+                        m_active_input_window = window.make_weak_ptr();
+                    }
                 }
                 return IterationDecision::Break;
             }
@@ -754,6 +772,7 @@ void WSWindowManager::process_mouse_event(WSMouseEvent& event, WSWindow*& hovere
             event_window_with_frame = &window;
             return IterationDecision::Break;
         });
+    }
 
     if (event_window_with_frame != m_resize_candidate.ptr())
         clear_resize_candidate();

Servers/WindowServer/WSWindowManager.h

@@ -220,6 +220,7 @@ private:
     WeakPtr<WSWindow> m_active_window;
     WeakPtr<WSWindow> m_hovered_window;
     WeakPtr<WSWindow> m_highlight_window;
+    WeakPtr<WSWindow> m_active_input_window;
 
     WeakPtr<WSWindow> m_drag_window;
     Point m_drag_origin;