[Update: This bug was first reported for HDRP/Compositor, but it can be reproduced in any Editor Window with a long title.]

Steps to repro:
1. Add a dummy window with a long title in your project. Example code:
class TestWindow : EditorWindow
{
static TestWindow s_Window;

[MenuItem("Window/Test Window", false, 10400)]
static void Init()
{
s_Window = (TestWindow)EditorWindow.GetWindow(typeof(TestWindow));
s_Window.titleContent = new GUIContent("Really Loooong Title (Preview)");
s_Window.Show();
}
}

2. Windows > Test Window
3. Docked the window > Over the Mouse on the window.
4. Close the Window(Using mouse middle-click).

Actual Result:
Window tooltip does not disappear.

Expected Result:
Window tooltip disappears.

Original text:

Graphics Compositor tooltip is present after closing compositor window.

Steps to repro:
1. Create a new HDRP project.
2. Windows > Render Pipeline > Graphics Compositor.
3. Docked the window > Over the Mouse on the window.
4. Close the Window(Using mouse middle-click).

Actual Result:
Graphics Compositor tooltip does not disappear.

Expected Result:
Graphics Compositor tooltip disappears.

Reproducible in:
2020.2.0b7, 2020.2.0a20 with Package Version 10.0.0-preview.27.

Note: Tooltip is available from Package version 10.0.0-preview.27.

Environment:
Occurring only on Windows 10.