So this wraps a non-menu-button toolbarbutton. I'm not familiar with this code, but at a glance, it looks like the issue is that the label/wrap used to be inherited from the parent element into the inner /nested toolbarbutton's via the `inheritedAttributes` which the c-c binding elides into its own inheritedAttributes. Now that the label isn't inherited via `inheritedAttributes` anymore, but via custom code in `attributeChangedCallback`, which calls `this.getAttribute()` to get the label value, we don't inherit the label/wrap from the "parent"/"outer" menu-button toolbarbutton into the inner toolbarbutton's text anymore. I suspect adding `label,wrap` to https://searchfox.org/comm-central/rev/89c30dfce0f903bcab7e25e9a9f24f39988ef40c/mail/base/content/toolbarbutton-menu-button.js#25 so we inherit those from the outer to the inner toolbarbutton would fix this issue.
Bug 1641514 Comment 2 Edit History
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So this wraps a non-menu-button toolbarbutton. I'm not familiar with this code, but at a glance, it looks like the issue is that the label/wrap used to be inherited from the parent element into the inner /nested toolbarbutton's `.toolbarbutton-text` via the `inheritedAttributes` which the c-c binding elides into its own inheritedAttributes. Now that the label isn't inherited via `inheritedAttributes` anymore, but via custom code in `attributeChangedCallback`, which calls `this.getAttribute()` to get the label value, we don't inherit the label/wrap from the "parent"/"outer" menu-button toolbarbutton into the inner toolbarbutton's text anymore. I suspect adding `label,wrap` to https://searchfox.org/comm-central/rev/89c30dfce0f903bcab7e25e9a9f24f39988ef40c/mail/base/content/toolbarbutton-menu-button.js#25 so we inherit those from the outer to the inner toolbarbutton would fix this issue.