Sorry for being PITA customer these days, but I'm trying to find my way out, and I guess I'm bit excited to play with DOpus ^^
Anyway, I just figured out that the default keyboard combinations Ctrl+Left, Ctrl+Right are used for Previous/Next Tab respectively. However, in thumbnail mode this effectively means you cannot select and move in horizontal direction while in Thumbnail mode.
I'm Ctrl+Tab user hence I can switch, but this somehow makes not okay for default installation. Or am I wrong?
You can turn off (or change) the next/prev tab hotkeys under Settings > Customize Toolbars > Keys.
(Help > Keyboard Map also goes to the same place.)
I did that already. My remark was more wrt whether is it not good idea to leave off the Ctrl+Left/Right from the default install (there is already Ctrl+PgUp/Down alternatives).
Just to be pedantic 
You can use Shift+Left/Right to "select and move" in thumbnails mode. Ctrl+Left/Right would let you move to start a new selection range while keeping the current selection.