Steps to reproduce:

1. Open attached project (tilemap-gone.zip)
2. Open 'TilemapTest' scene
3. Open 'Tile Palette' window, choose 'Rocks' tile palette and select one of the tiles to paint with
4. Click anywhere outside the tilemap (select some setting in Inspector or Hierarchy, or even click on some other app)
5. Then quickly click on the tilemap
6. Notice it registers the click as a mouse being dragged and draws a line (see attached gifs)

Happens on all Grid types (Rectangle, Hex, Isometric).

Additional Note: Issue is easier to reproduce on Mac Editor.

Reproduced with: 2017.4.7f1, 2018.2.6f1, 2018.3.0a11