The docs of TestCase::mouseDrag say "Note: this method does not imply a drop action, to make a drop, an additional mouseRelease(item, x + dx, y + dy) is needed."

However, you can find that exact line at the end of the function.

Additionally, it says "If \a delay is specified, the test will wait for the specified amount of milliseconds before releasing the button."

However, that delay is also used when pressing the button, and before each of the intermediate moves.