To get ready:
1. Open project attached (New Unity Project.zip)
2. Open Window "test" from Test -> Test

First issue - Button is marked as pressed when dragging back on button. (Look at gif named "First problem.gif" for comparison)
Reproduction steps:
1. Press on Button.
2. Drag outside of window.
3. Drag on button.

Expected result: Button will not be marked as pressed.
Actual result: Button is marked as pressed.

Note: If you drag button on other object in Hierarchy and back on button, it will not be marked as pressed.

Second problem - Button is marked as pressed, when drag was released in window, below button.
Reproduction steps:
1. Press on Button.
2. Drag below to an empty area.
3. Release it.

Expected result: Button will not be marked as pressed.
Actual result: Button is marked as pressed.

Third issue - Button starts using drag when moving window too fast.
Reproduction steps:
1. Press on top of window.
2. Drag it up and down.

Expected result: Window will be moved.
Actual result: Window stops moving, button starts using dragging.