This is essentially caused by the nonsensical removal of message header toolbar customization, but in spite of that, it's also a bug in it's own right, namely that when users reduce the width of the TB window and hence the width of msg preview also gets reduced, the toolbar doesn't scale. When the entire toolbar cannot fit, it should scale (i.e. not cause the entire layout to break) and either - expand to two rows - or show a [>>] disclosure button to access the off-screen buttons.
Bug 1669063 Comment 2 Edit History
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This is essentially caused by the nonsensical removal of message header toolbar customization, but in spite of that, it's also a bug in it's own right, namely that when users reduce the width of the TB window and hence the width of msg preview also gets reduced, the toolbar doesn't scale. When the entire toolbar cannot fit, it should scale (i.e. not cause the entire layout to break) and either - expand to two rows, or - show a [>>] disclosure button at the right end to access the off-screen buttons.
This is essentially caused by the nonsensical removal of message header toolbar customization, but in spite of that, it's also a bug in it's own right, namely that when users reduce the width of the TB window and hence the width of msg preview also gets reduced, the toolbar doesn't scale. When the entire toolbar with all text+icon buttons cannot fit, it should scale (i.e. not cause the entire layout to break) and either - Allow the labels to truncate, and/or switch to icons-only when there's not enough horizontal space for text+icons - show a [>>] disclosure button at the right end to access the off-screen buttons, or - expand to two rows