I noticed this issue just today, on my 2013 Mac.
Join this experience- 2D game (for testing) - Roblox
Scale the roblox window down.
Move your character right, and without letting go of the “D” key, slowly increase your roblox window, or just hold the edge of the window.
While holding or scaling the window, let go of The “D” key, or whatever movement you are performing.
Notice that the Character continues to move, even though you aren’t pressing anything.