A shadow root scopes selectors, not layout. `position:fixed` in mail CSS is
still positioned against the viewport, and the containment meant to stop that
sat inside the shadow root as `.ihm-email-root { contain: content }` — in the
same tree as the message's own <style>, which is inserted after it and simply
overrides it. Any sender could cover the entire window with markup of their
choosing, inside our own origin: a ready-made place to ask for a password.
Verified in a browser: the message rendered at exactly the viewport size with
the maximum z-index.
Moving the containment onto the shadow host does not fix it — mail CSS reaches
the host through `:host`, and an `!important` there beats an `!important` from
the app's own stylesheet, because importance reverses tree order in the
cascade. An ancestor of the host is the one thing mail CSS has no selector
for, so the control goes on .message-body. `layout` rather than `paint`: it
makes the element a containing block for fixed descendants without clipping
tall messages.
The sanitizer now also turns fixed and sticky positioning static and defangs
`:host`, as a second line that does not depend on one CSS declaration.
Checked end to end against the real stylesheet afterwards: the same message
renders 1678x112 instead of 1720x1279.
Messages render on a white card in every theme. That is deliberate for mail
that styles itself, but #4 points out the case it gets wrong: a message with
no styling of its own has nothing worth preserving, and flashing white at
someone reading in the dark is a real cost.
Appearance gains a switch under the theme cards, off by default so the
current behaviour is unchanged. With it on, HTML mail that declares no
colours follows the app theme; mail that sets a background or text colour
still gets the light card it was designed for, because half-darkening someone
else's design is worse than leaving it alone. Plain-text mail already
followed the theme and is untouched by the switch.
The themed palette is expressed in the app's own custom properties, which
cross the shadow boundary, so switching theme repaints open messages without
re-rendering them, and the accent-coloured link stays consistent. The host
element takes color-scheme: inherit so form controls and scrollbars inside a
message match too.
htmlDeclaresColors covers bgcolor attributes, <font color>, and colour or
background declarations in style attributes and <style> blocks, while
ignoring near-misses like border-color and ?color= in a URL.
Closes#4
jcoffey-dev