Proposal: Clarify the scale parameter for tabs.captureVisibleTab

Following the recent implementation of the rect parameter in tabs.captureVisibleTab in Chrome (see w3c/webextensions#850), we also added the scale parameter, which is already supported by Firefox.

However, we've realized that the behavior of the scale param differs between Chrome and Firefox as reported here in this issue tracker.

Firefox (top row): scale controls the rendering resolution of the captured image. A developer can capture the same rectangular area at different resolutions just by changing the scale value.

Chrome (bottom row): scale adjusts the dimensions of the capture rectangle itself, not the resolution of the content within it.

We're unlikely to handle capturing at a higher resolution like what Firefox does in the near future as we do not have a way to re-render content at a different DPI of the rendering surface itself.

We wanted to double check to see which is the intended behavior (should scale be controlling resolution or dimensions?).

Start with the tabs.captureVisibleTab specification and its scale parameter, then compare the Chrome report and Firefox documentation linked in the issue. Done means reaching agreement on whether scale controls resolution or capture dimensions and updating the specification to state that behavior clearly.