Bug 1749758 - Disable session store for the Firefox Suggest toggles/c…

…heckboxes in about:preferences. r=Gijs,preferences-reviewers

Please see bug 1749758 comment 6 for background. In summary, session store is
restoring the states of the Firefox Suggest `<html:input>` checkboxes in
about:preferences#privacy, which intermittently happens **after** their states
are initialized by the preferences code. If a checkbox's session store state
does not match the value of its underlying pref, then the checkbox and pref both
end up with the wrong value because session store sets the wrong checked state
of the checkbox, which fires an input event, which then updates the pref.

This can only happen when the pref value is changed outside of
about:preferences, which is indeed the case for the Firefox Suggest pref
discussed in the bug.

To fix it, I tried adding `autocomplete=off` to each input, but unfortunately
that does not work because `input[type=checkbox]` is specifically not allowed to
opt out of autocomplete/session store. The code path for restoring input
checkboxes is:

# [CollectInputElement](https://searchfox.org/mozilla-central/rev/3de56eb5f266f523340e739ae1b53258e0a95dfe/toolkit/components/sessionstore/SessionStoreUtils.cpp#580)
  # CollectInputElement [calls](https://searchfox.org/mozilla-central/rev/3de56eb5f266f523340e739ae1b53258e0a95dfe/toolkit/components/sessionstore/SessionStoreUtils.cpp#603) [nsContentUtils::IsAutocompleteEnabled](https://searchfox.org/mozilla-central/rev/3de56eb5f266f523340e739ae1b53258e0a95dfe/dom/base/nsContentUtils.cpp#1070)
    # nsContentUtils::IsAutocompleteEnabled [calls](https://searchfox.org/mozilla-central/rev/3de56eb5f266f523340e739ae1b53258e0a95dfe/dom/base/nsContentUtils.cpp#1075) [HTMLInputElement::GetAutocomplete](https://searchfox.org/mozilla-central/rev/3de56eb5f266f523340e739ae1b53258e0a95dfe/dom/html/HTMLInputElement.cpp#1394)
    # HTMLInputElement::GetAutocomplete [calls](https://searchfox.org/mozilla-central/rev/3de56eb5f266f523340e739ae1b53258e0a95dfe/dom/html/HTMLInputElement.cpp#1395) [HTMLInputElement::DoesAutocompleteApply](https://searchfox.org/mozilla-central/source/dom/html/HTMLInputElement.cpp#6365)
    # HTMLInputElement::DoesAutocompleteApply [returns false](https://searchfox.org/mozilla-central/rev/3de56eb5f266f523340e739ae1b53258e0a95dfe/dom/html/HTMLInputElement.cpp#6398) for FormControlType::InputCheckbox
  # Back in nsContentUtils::IsAutocompleteEnabled, the `autocomplete` string is empty because HTMLInputElement::DoesAutocompleteApply did not set it to anything, so then the method [checks](https://searchfox.org/mozilla-central/rev/3de56eb5f266f523340e739ae1b53258e0a95dfe/dom/base/nsContentUtils.cpp#1078) if the input has a form and sets `autocomplete` to the value of the form's autocomplete attribute
  # nsContentUtils::IsAutocompleteEnabled then returns false iff `autocomplete` is "off"
# Back in CollectInputElement, if nsContentUtils::IsAutocompleteEnabled
   returned true, then it continues and checks a couple of other things, but at
   that point there's no way for the input checkbox to opt out of
   autocomplete/session store.

So, the only way to disable autocomplete seems to be to associate the input with
a form and set `autocomplete=off` on the form, so that's what I've done.

It would be simpler to make these XUL checkboxes instead, which AFAICT don't
participate in session store, but these checkboxes need the toggle-switch
styling.

Finally, this is a problem for all `<html:input>`s used in about:preferences. I
didn't update any others because I want to keep this scoped for uplift, and
maybe the other input checkboxes should be XUL checkboxes anyway?

Differential Revision: https://phabricator.services.mozilla.com/D136054

browser/components/preferences/privacy.inc.xhtml

@@ -651,6 +651,10 @@
   <checkbox id="enginesSuggestion" data-l10n-id="addressbar-locbar-engines-option"
             preference="browser.urlbar.suggest.engines"/>
   <vbox id="firefoxSuggestContainer" hidden="true">
+    <!-- This form[autocomplete="off"] is necessary to prevent the inputs from
+         participating in session restore. autocomplete="off" is ignored on the
+         inputs themselves. -->
+    <html:form id="firefoxSuggestForm" autocomplete="off" hidden="true"/>
     <vbox class="firefoxSuggestOptionBox">
       <hbox align="center">
         <label id="firefoxSuggestNonsponsoredLabel"
@@ -660,6 +664,7 @@
                     type="checkbox"
                     class="toggle-button firefoxSuggestToggle"
                     preference="browser.urlbar.suggest.quicksuggest.nonsponsored"
+                    form="firefoxSuggestForm"
                     aria-labelledby="firefoxSuggestNonsponsoredLabel"
                     aria-describedby="firefoxSuggestNonsponsoredDescription"/>
       </hbox>
@@ -676,6 +681,7 @@
                     type="checkbox"
                     class="toggle-button firefoxSuggestToggle"
                     preference="browser.urlbar.suggest.quicksuggest.sponsored"
+                    form="firefoxSuggestForm"
                     aria-labelledby="firefoxSuggestSponsoredLabel"
                     aria-describedby="firefoxSuggestSponsoredDescription"/>
       </hbox>
@@ -692,6 +698,7 @@
                     type="checkbox"
                     class="toggle-button firefoxSuggestToggle"
                     preference="browser.urlbar.quicksuggest.dataCollection.enabled"
+                    form="firefoxSuggestForm"
                     aria-labelledby="firefoxSuggestDataCollectionLabel"
                     aria-describedby="firefoxSuggestDataCollectionDescription"/>
       </hbox>

browser/themes/shared/preferences/privacy.css

@@ -332,7 +332,7 @@
   margin-block-start: 11px;
 }
 
-.firefoxSuggestOptionBox:first-child {
+.firefoxSuggestOptionBox:first-of-type {
   /* Similar here: Make the apparent vertical space between the last checkbox
      and first option box the same as elsewhere. */
   margin-block-start: 5px;