If you open chrome and perform the pinch gesture, what happens? Does the whole interface become bigger/smaller?
No, only the page content gets magnified, as one would expect. And it is some feature beside the magnifier as well, because the magnifier stays at 100% all the time.
EDIT: After a quick research, I found the following code
const webFrame = require("electron").webFrame;
webFrame.setVisualZoomLevelLimits(1, 3);
Which allows me to zoom on the whole interface (as if the sidebars and navigation bars were parts of the content, too). Now just to figure out how to specify that those should have fixed size and position…