The overall behavior is perhaps a bit easier to see with testcase 2 from bug 1912854: https://bug1912854.bmoattachments.org/attachment.cgi?id=9419991 * In Firefox, the giant fixed-pos cyan element (with an orange border) stays the same "smaller" size as you start to scroll the dynamic toolbar out of view, and then it snaps to be full-size when the toolbar fully hides. We reverse this process exactly when you start to scroll the dynamic toolbar back into view -- first the giant cyan element snaps to be the "smaller" size and gets vertically centered with space above and below it, and that space gradually shrinks as you scroll the dynamic toolbar fully into view. * In Chrome, the giant cyan element is **always snapped to the top of the screen** (i.e. the "top:0" is paramount), but otherwise behaves similarly to Firefox. It stays the same "smaller" size (but snapped to the top) as the toolbar starts to hide; this means some space is created below it (at the bottom of your screen) as the toolbar is animating away. Then it snaps to the larger size once the toolbar is fully hidden. If you then scroll back up to reveal the toolbar, Chrome keeps the cyan element at the larger size (meaning its bottom edge gets clipped) until the toolbar is fully in view.
Bug 1917978 Comment 1 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
The overall behavior is perhaps a bit easier to see with testcase 2 from bug 1912854: https://bug1912854.bmoattachments.org/attachment.cgi?id=9419991 * In Firefox, the giant fixed-pos cyan element (with an orange border) stays the same "smaller" size as you start to scroll the dynamic toolbar out of view, and then it snaps to be full-size when the toolbar fully hides. We reverse this process exactly when you start to scroll the dynamic toolbar back into view -- first the giant cyan element snaps to be the "smaller" size and gets vertically centered with space above and below it, and that space gradually shrinks as you scroll the dynamic toolbar fully into view. Throughout this whole process, the cyan element is vertically centered in the viewport (so there's blank space above it and below it for all of the intermediate states). * In Chrome, the giant cyan element is **always snapped to the top of the screen** (i.e. the "top:0" is paramount), but otherwise behaves similarly to Firefox. It stays the same "smaller" size (but snapped to the top) as the toolbar starts to hide; this means some space is created below it (at the bottom of your screen) as the toolbar is animating away. Then it snaps to the larger size once the toolbar is fully hidden. If you then scroll back up to reveal the toolbar, Chrome keeps the cyan element at the larger size (meaning its bottom edge gets clipped) until the toolbar is fully in view.
The overall behavior is perhaps a bit easier to see with testcase 2 from bug 1912854: https://bug1912854.bmoattachments.org/attachment.cgi?id=9419991 * In Firefox, the giant fixed-pos cyan element (with an orange border) stays the same "smaller" size as you start to scroll the dynamic toolbar out of view, and then it snaps to be full-size when the toolbar fully hides. We reverse this process exactly when you start to scroll the dynamic toolbar back into view -- first the giant cyan element snaps to be the "smaller" size and gets vertically centered with space above and below it, and that space gradually shrinks as you scroll the dynamic toolbar fully into view. Throughout this whole process, the cyan element is vertically centered in the instantaneous "user-exposed" viewport (so there's blank space above and below the cyan element for all of the intermediate states where it's the "smaller" size and the user-exposed viewport is somewhat larger). * In Chrome, the giant cyan element is **always snapped to the top of the screen** (i.e. the "top:0" is paramount), but otherwise behaves similarly to Firefox. It stays the same "smaller" size (but snapped to the top) as the toolbar starts to hide; this means some space is created below it (at the bottom of your screen) as the toolbar is animating away. Then it snaps to the larger size once the toolbar is fully hidden. If you then scroll back up to reveal the toolbar, Chrome keeps the cyan element at the larger size (meaning its bottom edge gets clipped) until the toolbar is fully in view.
The overall behavior is perhaps a bit easier to see with testcase 2 from bug 1912854: https://bug1912854.bmoattachments.org/attachment.cgi?id=9419991 * In Firefox, the giant fixed-pos cyan element (with an orange border) stays the same "smaller" size as you start to scroll the dynamic toolbar out of view, and then it snaps to be full-size when the toolbar fully hides. We reverse this process exactly when you start to scroll the dynamic toolbar back into view -- first the giant cyan element snaps to be the "smaller" size and gets vertically centered with space above and below it, and that space gradually shrinks as you scroll the dynamic toolbar fully into view. Throughout this whole process, the cyan element seems to be **vertically centered** in the instantaneous "user-exposed" viewport (so there's blank space above and below the cyan element for all of the intermediate states where it's the "smaller" size and the user-exposed viewport is somewhat larger). * In Chrome, the giant cyan element is **always snapped to the top of the screen** (i.e. the "top:0" is paramount), but otherwise behaves similarly to Firefox. It stays the same "smaller" size (but snapped to the top) as the toolbar starts to hide; this means some space is created below it (at the bottom of your screen) as the toolbar is animating away. Then it snaps to the larger size once the toolbar is fully hidden. If you then scroll back up to reveal the toolbar, Chrome keeps the cyan element at the larger size (meaning its bottom edge gets clipped) until the toolbar is fully in view.