I can now reliably state (after testing) that scaling the tip down from 1.0 after you've added something to a line that involves multiple columns has a chance to produce "trimmed" strings (and possibly textures, dunno, perhans Xinh can enlighten us on that).
I can't really explain why either.
What I can tell you though, is that each fontstring in a tooltip (both left and right sides) are parented to the tooltip, and each fontstring is setpointed at the TOPLEFT to the fontstring above it to its BOTTOMLEFT (if I remember correctly).
Where there are sockets, there is an X offset for that line's offset from the previous line to account for the icon, and then a -X offset when it goes back to lines without an icon.
Now the height of these fontstrings aren't fixed, the width is automated based on the tooltip's width, but when you scale an object with a fontstring in it, the actual font displayed doesn't get scaled, the game calculates the "closest" font size to shrink the font down to to display it correctly.
You can observe this behavior easily, Have a fontstring at 1.0 scale, and have its fontsize at say 16. SLowly shrink the scale down, 0.99 then 0.98, etc, the displayed text will not shrink in size until the scale is enough to actually make the font size become 15 (and is rendered as 15), so its more of a step ladder effect.
My guess of course, is that this is to prevent aliasing of fonts or something, and whatever scale the fontstring is at, it remains readable by rendering it at the actual font size it is to be displayed at after accounting for scale.
I'm currently waiting for click and scroll support to since these are the features I need in most addons using extended tooltips :/
But I do understand that for most normal display tooltips these aren't needed.
Now if a tooltip needs to be that big that it needs scrolling, it isn't really a tooltip anymore eh? Might be better to create your own special frame for it. :D
Can you guys provide some insight into the CPU and memory usage of LibTooltip (by disembedding it)? Tablet used up a crazy amount of memory and CPU (see PerformanceFu), I'll like to see some numbers as comparison for creating say the same tooltip in Tablet versus the same tooltip in LibTooltip.