Use previous width rather than height for Picture-in-Picture initial size

[aria1]

Since the feature was first implemented, vertical video on desktop has become a thing. When resizing a PiP window for a horizontal video and then using the same height for a vertical video, the experience is worse than if both used the same width instead.

It can be an about:config setting or a about:settings setting if you conclude the default behaviour is still ideal, but also see that some users would prefer going by width.

A third option of using some dynamic size derived from the height and width is also possible. For example total pixels. Please just revisit the decision.

[Jon]

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