Allow smooth resizing when multi resizing windows

Bug: 954510
Test: covered by unittest
Change-Id: I954bd6b9b928b5d765e0d646cf77ebb6fca3745a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1574285Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652553}

ash/wm/workspace/multi_window_resize_controller_unittest.cc

@@ -278,6 +278,17 @@ TEST_F(MultiWindowResizeControllerTest, Drag) {
   EXPECT_TRUE(IsShowing());
   EXPECT_EQ(gfx::Rect(0, 0, 110, 100), w1->bounds());
   EXPECT_EQ(gfx::Rect(110, 0, 100, 100), w2->bounds());
+
+  // It should be possible to move 1px.
+  bounds = resize_widget()->GetWindowBoundsInScreen();
+
+  generator->MoveMouseTo(bounds.x() + 1, bounds.y() + 1);
+  generator->PressLeftButton();
+  generator->MoveMouseBy(1, 0);
+  generator->ReleaseLeftButton();
+
+  EXPECT_EQ(gfx::Rect(0, 0, 111, 100), w1->bounds());
+  EXPECT_EQ(gfx::Rect(111, 0, 100, 100), w2->bounds());
 }
 
 // Makes sure three windows are picked up.

ash/wm/workspace/workspace_window_resizer.cc

@@ -905,6 +905,10 @@ bool WorkspaceWindowResizer::UpdateMagnetismWindow(
   const std::vector<aura::Window*>& children = container->children();
   for (auto i = children.rbegin();
        i != children.rend() && !matcher.AreEdgesObscured(); ++i) {
+    // Ignore already attached windows.
+    if (base::ContainsValue(attached_windows_, *i))
+      continue;
+
     wm::WindowState* other_state = wm::GetWindowState(*i);
     if (other_state->window() == GetTarget() ||
         !other_state->window()->IsVisible() ||