non-clickable window partially obscuring firefox

Submitted by Alistair Buxton @ali1234

Link to original bug (#15291)

Description

I have a firefox window open and a small area of it cannot be clicked on. xwininfo on the unclickable region says this:

xwininfo: Please select the window about which you
          would like information by clicking the
          mouse in that window.

xwininfo: Window id: 0x8f6b18 (has no name)

  Absolute upper-left X:  1871
  Absolute upper-left Y:  25
  Relative upper-left X:  1871
  Relative upper-left Y:  25
  Width: 389
  Height: 149
  Depth: 32
  Visual: 0xc2
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x400003b (not installed)
  Bit Gravity State: StaticGravity
  Window Gravity State: StaticGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +1871+25  -2220+25  -2220-1026  +1871-1026
  -geometry 389x149+1871+25

Attached screenshot shows the non-clickable region.

I'm not sure if this is a firefox bug or an xfwm bug. Note that the non-clickable window seems to be attached to one specific firefox window, but sometimes extends outside it to obscure other windows.

Attachment 8415, "screenshot":