@
theducks - ah-ha, I think you're referring to a column like author or tags
For some reason I homed in on columns with a defined set of values

The drop down for those columns is full width, even if the column is narrow - that would be because the max width can predetermined.
To do that with a column that has dynamic values (like tags or author) one would probably need to squirrel away the length of the longest value and build the drop down to that width.
I suspect the only answer is 'user must make the column wider' - but Kovid or chaley might come up with something.
The tooltip is sized to the length of the cell under the pointer, so I don't see how it helps. If the Author is "Violet Bonham Carter" the tool tip is 20 chars wide, if its "Homer" its 5. If it's the tags column and the cell contains values imported from Amazon the tooltip could overlap into
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