Okay, I have two more suggestions, as shown in the image. The parts underlined in red are where I think improvements should be made:
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The header is too wide and takes up too much space; I suggest reducing its height slightly.
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Compared to the right side, the left side has a larger width and denser content, while the right side has a smaller width. This lack of balance becomes obvious when a colored background is applied. It becomes more uncomfortable on larger monitor screens.
I suggest using a light gray for the background color, increasing the width of the right sidebar, and adding some expandable function buttons. I found a reference case with a similar style:
Reference https://mail.google.com/chat