Bug 267201

Created attachment 469327 [details] The test case is containing a simple page allowing to enter fullscreen mode on an image element. Description Without having a viewport meta tag defined, the zoom/scale factor is far off after entering fullscreen mode. The issue is most obvious in portrait mode, where the element appears almost outside of the display area, but landscape mode is affected too. Attempts to pinch to correct zoom factor often lead to exiting fullscreen mode. We are providing a javascript framework, therefore the viewport meta tag is not under our control. Test case The test case is containing a simple page allowing to enter fullscreen mode on an image element. The page does not contain a viewport meta tag. We are facing the same issue with div elements containing video. Assuming that the issue does not depend on the type of element. iOS/Safari settings * Accessibility/Zoom: Off (default) * Safari/Page Zoom: 100% (default) * Safari/Advanced/Feature Flags: defaults, except for Fullscreen API: On * Mobile vs. Desktop Mode: both affected Steps to reproduce 1) Page load, portrait mode recommended 2) Tap button to enter fullscreen mode 3) Check if image is displayed centered and completely 4) Exit fullscreen mode 5) Apply a tiny bit of pinch zoom, in and out to undo 6) Tap button to enter fullscreen mode 7) Check if image is displayed centered and completely Test result iPhone 13, iOS 17.3 beta (21D5026f) * no issue in step 3), but always in step 7) Test result iPhone 12 mini, iOS 17.3 beta (21D5026f) * always issues in step 3) and 7) Test result iPhone 11, iOS 17.2.1 * always issues in step 3) and 7) Observations We have checked window.visualViewport.scale values before and after entering fullscreen mode. iPhone 13, iOS 17.3 beta (21D5026f), portrait mode In case of success: before ~0.38 after ~0.38 In case of failure: before ~0.38 after 1.0 We have further noticed that the amount of the scaling error in fullscreen mode is quite constant and does rather depend on the ratio between default scale and 1.0, e.g. ~0.38/1.0. Even if applying maximum zoom before entering fullscreen mode, the appearance will be the same. We assume this is why the impact of the issue is smaller in landscape mode, because the default scale is much closer to 1.