As reported by @bb in rP21142, the right-side of some tooltips don't appear to have the correct margin/padding on the right-hand side edge.
On close inspection, this appears to affect all tooltips with text short enough that line-wrapping does not happen, with wrapped tooltips looking okay.
Inspecting the code, it appears that when each word is measured, the space between the current word and the next is included in the result.
The solution implemented below deducts the width of the space from the calculated width of the last word in a line where wrapping occurs; then adds the BufferZone variable a second time to give a right as well as a left internal margin.