Bug 1852849 - Make...

Bug 1852849 - Make `HTMLEditor::AutoInlineStyleSetter::ExtendOrShrinkRangeToApplyTheStyle` recompute common ancestor of the range if it updates the range r=m_kato a=RyanVM

In this case, `<font face="monospace">abc []<br></font>`, the range applying
new style will be extended as `<font face="monospace">abc [<br>}</font>` first.
https://searchfox.org/mozilla-central/rev/29bdf6ff9965a647c6f64d63fed2b5bd094532c7/editor/libeditor/HTMLStyleEditor.cpp#1944

Then, the start point should be shrunken and the range should become
`<font face="monospace">abc {<br>}</font>`.
https://searchfox.org/mozilla-central/rev/29bdf6ff9965a647c6f64d63fed2b5bd094532c7/editor/libeditor/HTMLStyleEditor.cpp#1977-1978

However, `commonAncestor` is still the text node because it's not updated
after extending the range to include the `<br>`.  Then,
`AutoInlineStyleSetter::GetNextEditableInlineContent` fails to get `<br>`
from the text node.
https://searchfox.org/mozilla-central/rev/29bdf6ff9965a647c6f64d63fed2b5bd094532c7/editor/libeditor/HTMLStyleEditor.cpp#1576-1578

Finally, the unexpected range computation will reach here with the text editor
and adjust the start of the range to start of the text node.
https://searchfox.org/mozilla-central/rev/29bdf6ff9965a647c6f64d63fed2b5bd094532c7/editor/libeditor/HTMLStyleEditor.cpp#420-423

Therefore, the new text which the new style should be applied is jumped to
start of the text node.

Differential Revision: https://phabricator.services.mozilla.com/D190028