The above is a cropped screenshot of mIRC, where I have added red and green outlines.
When the mouse hovers over the green area -- where the URL is displayed -- the mouse cursor does NOT change and clicking in this area does NOT launch the URL.
When the mouse hovers over the red outline, the mouse cursor changes to the usual "finger pointing" cursor and double-clicking on this region launches the URL.
You can move the mouse cursor around all you want, and the behavior remains until you scroll the window up or down, at which point it will correct itself. Right-clicking also seems to cause the window to fix (by shifting all text up one line).
It's just an issue with mIRC not redrawing the window correctly under some circumstances when scrolling. It only happens occasionally, but I've reproduced it at least 4 times today.