Thank you for taking the time to reply. The CSS hides the mobile menu panel from showing but the menu/hamburger is not the issue, it is the Slider widget. If I use the CSS, instead of filling the whole mobile screen the mobile menu panel becomes 90% width. (I have removed the CSS)
The Slider widget is causing alignment problems for this specific page where it is used, compared to other posts where Slider widget is not used. I have also deleted the slider and the problem disappears.
In mobile mode, the slider seems to be changing the page container width, forcing it wider than my site settings for the content. And while it is keeping the left padding, the right padding is removed for the post’s contents, but it applies to the menu. This is why the mobile menu panel is showing but the post content is stretched to the right of the screen with no padding.
Even if I set the slider’s width to eg 200px and set the mobile margins/paddings, it still displays full width on mobile with no right padding/margin.