When plot outputs appear inside scaled regions of the DOM, Shiny reports the output area in absolute pixels (i.e. the size of the output area after the scaling is applied). The returned image, however, ends up being smaller than desired because it doesn't take into account the scaling.

You can see this happening in the superzip example.

I've applied a red outline to the plot output area and a blue outline to the actual image. Ideally these would overlap and the image would take up the full output area.

.shiny-plot-output {
    outline: 1px solid red;
}

.shiny-plot-output > img {
    outline: 1px solid blue;
}

In this example, on app load, the histCentile output reports size 259 x 180px and receives an image with those dimensions. But the output appears in an absolute panel (#controls) that has zoom: 0.9 applied. Those are indeed the absolute dimensions of the #histCentile container after the browser has scaled the output container. Once placed, the image ends up with the absolute size of 233.09 x 162px. The image should instead be 287.78 x 200px (taking into account the scaling) to end up with the correct size.