Allow selecting stacks by clicking between samples in the sample graph (should select closest sample)

[mstange]

Profile: https://share.firefox.dev/412IfVv

When there are gaps between samples, clicking in those gaps no longer selects any stacks. It would be nice if it selected the stack of the closest sample.

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In this profile, try to quickly answer the question "where are we blocking during the large 0% CPU gap near the start of the profile?"

You can't click anything in the activity graph because there's nothing to click on during the 0% CPU time. So you'll have to click samples. And with the gaps between the samples, this requires rather accurate mouse movements.
(Or you could switch to the stack chart and scroll down, but I usually forget about that.)

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