Blender Git Loki

Fix T67756: File drag starts on file browser open

When the file browser was opened (from a temporary window since the file
browser redesign) using a button in the UI, under certain conditions
moving the mouse would trigger files to be dragged.

Note that this has been an issue before the new file browser design was
introduced, although under slightly different conditions.

Steps to reproduce:
* With factory settings, press F12
* Open a different image in the appearing Image Editor (not Render
Result)
* Make sure the window is not maximized
* Press N to open the side bar, open Image tab
* Click the folder icon there to change the image
* Change to thumbnail display type in the appearing file browser
* Cancel, click the folder icon again
Moving the mouse now would start dragging files in most cases.

The same issue could be reproduced in a similar way when installing
lights/MatCaps or HDRIs through Preferences -> Lights -> Install...