Ok... I remember what's going on here and why it is this way.
Centering the list and conversation in the space available (100% - width of nav) doesn't quite look right as the nav doesn't look like it is as wide as it is.
I add some right padding on the conversation to compensate for this but not on the list (as it was harder to apply to the list without breaking things)
The end result is the jump you're seeing with the nav is open :(
Bottom line:
I half fixed the centering of content with the nav open
I should probably remove my right padding on the conversation OR figure out how to add it to the list correctly
When the main menu left side panel is open, there's a visible jump when switching between email list and reading view:
Simplify v.2.3.7 - Configuration: aAX ca4 caE caI caL cfC cfO cfQ cfR cfU composeActions dateGroup defaultTheme inInbox inList invertAddons invertCompose izBgDefault lightTheme listWidthLg lowDensity matchFontSize minimizeChat msgWidthSm navOpen newUI simplify - System: Mozilla/5.0 (X11 Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/89.0.4389.90 Safari/537.36 - Window: 1920 x 1002 (100pct zoom) - Language: en