fix: #4116

Hi @yexingsha, this is what this PR looks like, please check if I missed anything:

MacOS:

Windows:

On macOS, the text and icon are positioned 1px too high within the button (it's more obvious with the library icon). And both the spacing and padding are still a bit too large. Seems like the extent of the button is about 2px wider than its optical width, so need to compensate for that:

The spacing between the icon and text can be reduced to 4px.

The header looks much better now at narrow width, but looks weird at larger width:

I think once the "Add to" button grows wide enough to fully display the text, both buttons should grow in width as the pane gets wider. And perhaps the split menu button should still align to center, even though it does go against convention.

Thanks. Done:

• the text and icon are positioned 1px too high within the button
• And both the spacing and padding are still a bit too large. Seems like the extent of the button is about 2px wider than its optical width, so need to compensate for that
• The spacing between the icon and text can be reduced to 4px.
• And perhaps the split menu button should still align to center, even though it does go against convention.
• I think once the "Add to" button grows wide enough to fully display the text, both buttons should grow in width as the pane gets wider

This looks great!