In the code of $:/core/ui/Buttons/save there is one line too many, which creates an unwanted space next to the save button if there is a button to the right of it in the EditToolbar. To remove it, just delete the last line of that code, after <<save-tiddler-button>>.
If anyone wants to see it, it happens even with the most recent empty.html on https://tiddlywiki.com/.
Just go in $:/ControlPanel > Appearance > Toolbars > EditToolbar and move $:/core/ui/Buttons/save above delete or cancel, then edit a tiddler and you can see the space. Then if you edit $:/core/ui/Buttons/save and you delete the last empty line, that space goes away.
\define save-tiddler-button()
\whitespace trim
<$fieldmangler><$button tooltip={{$:/language/Buttons/Save/Hint}} aria-label={{$:/language/Buttons/Save/Caption}} class=<<tv-config-toolbar-class>>>
<<save-tiddler-actions>>
<$list filter="[<tv-config-toolbar-icons>match[yes]]">
{{$:/core/images/done-button}}
</$list>
<$list filter="[<tv-config-toolbar-text>match[yes]]">
<span class="tc-btn-text"><$text text={{$:/language/Buttons/Save/Caption}}/></span>
</$list>
</$button></$fieldmangler>
\end
<<save-tiddler-button>>
HERE ONE LINE TOO MUCH
Nothing special, I don't know if it's already known, I searched a bit and couldn't find anything, so I thought I'd mention it here. (I'm no expert, if that space is actually wanted for some reason, my bad).