1690697 - Try to bound subframe displayport sizes more tightly

Status: NEW

(Core :: Panning and Zooming, defect, P3)

Description

[Botond Ballo [:botond]]

While investigating bug 1650183, I discovered that the limit on a the size of a subframe's displayport is larger than I expected.

We limit the size of the displayport base rect to the size of the root displayport, so in the case of a large-viewport subframe, this is potentially as large as the root viewport size + the root's displayport margins.

We then add margins for the subframe itself, resulting in a subframe displayport whose size is potentially as large as the root viewport + two sets of margins. With a displayport multiplier of 3.5 in the vertical direction, this means the subframe displayport can be ~6x as tall as the root viewport.

Expanding the base rect to be the root displayport instead of the root viewport was done in bug 1617427 to fix a checkerboarding problem, but I don't think we considered its impact on the bounding size of subframe displayports.

We should see if we can bound subframe displayport sizes more tightly than this, without regressing bug 1617427.