Bugzilla

Two more observations:
 (3) Even with "always enable zoom" turned off (the default), the tap-on-the-image action triggers visible scrollbars -- i.e. there's some content appearing outside of the viewport! If you scroll a bit, you can see that this outside-the-viewport content is the Accessible Caret (little blue pointed selection-circles).

 (4) If you turn off pref `layout.accessiblecaret.enabled_on_touch`, then this bug seems to entirely go away. No scrollbars appear, and I can't reproduce the surprise context menu or the insane zooming.

So my current theory is:
 - Tapping the image triggers a "selection" action with the accessible caret, for some reason.
 - We select a fullscreen fixed-pos element with the accessible caret, and this creates content (the caret itself) outside of the visible part of the viewport.
 - If always-enable-zoom is turned on, then this apparently expands the viewport size that the fixedpos element is filling, which grows the `height:100%;width:100%` fixed-pos element itself.
 - That, in turn, moves the selection-caret pointy-circles outward, which then grows the viewport more, etc etc. And we have a deathspiral outwards.

Two more observations:
 (3) Even with "always enable zoom" turned off (the default), the tap-on-the-image action triggers visible scrollbars -- i.e. there's some content appearing outside of the viewport! If you scroll a bit, you can see that this outside-the-viewport content is the Accessible Caret (little blue pointed selection-circles).

 (4) If you turn off pref `layout.accessiblecaret.enabled_on_touch`, then this bug seems to entirely go away. No scrollbars appear, and I can't reproduce the surprise context menu or the insane zooming.

So my current theory is:
 - Tapping the image triggers a "selection" action with the accessible caret, for some reason (maybe in part due to how Twitter is handling touch events per comment 9).
 - That "selection" action selects a 100%-sized fixed-pos element (covering the full viewport), using the accessible caret, and this creates content (the begin-selection/end-selection carets themselves) outside of the visible part of the viewport.
 - If always-enable-zoom is turned on, then this apparently expands the fixedpos element's containing block (by expanding the layout viewport, or something?), which grows the `height:100%;width:100%` fixed-pos element itself.
 - That, in turn, moves the selection-caret pointy-circles outward, which then grows the viewport more, etc etc. And we have a deathspiral outwards.