I think it's either your install or specific hardware... Haven't seen mention of either any-where.
I tried running a 'liveCD of Mint13 Xfce (no updates yet) on 3 different computers 1)old inspirion 1721 laptop, 2) ThinkPad x200, 3)ancient HP TC1100 tablet. (1st two(2) in 32 and 64 mode)
able to put launcher (a terminal) in lower left corner. With a FF session runing in right 1/2 of screen, double click launcher to activate. I can start typing imeidialy in terminal no matter where the mouse is. Can also 'shortcut key' alt-tab to change focus between terminal and browser! (no mouse movement at-all)
P.S. are you on 'LMinnt13-Xfce or LMDE-Xfce and also 32 or 64 bit version?