Reproduce the bug this way:

• Press left mouse button on an SDL window (turn mouse capturing on) and drag mouse outside the window.
• While still holding the left mouse button, press Win + Tab and select a different window using the keyboard (arrow buttons and Enter).
• Release the left mouse button.
• When we move the mouse over the SDL window again, the left mouse button seems to be still pressed (SDL_GetMouseState returns SDL_BUTTON_LMASK).
• Win + Tab and select the SDL window again (now window becomes highlighted) -> delayed mouse button up event arrives.