When dragging/moving messages the one you drag is deselected

[runboxdave]

When you select multiple messages and drag them to another folder, the one you actually put the mouse on to drag is deselected and not moved.

Describe the bug Moving multiple messages with drag and drop deselects the dragged message.

To Reproduce Steps to reproduce the behavior:

1. Go to https://runbox.com/app
2. Click to select 2 or more messages
3. Drag the subject line of either message to another folder
4. Observe that the dragged message is deselected and not moved

Expected behavior All selected messages should be moved.

Desktop (please complete the following information):

• OS: macOS
• Browser: Firefox

[gtandersen]

See also #1095.