Imagine you’ve tried turning off themes, custom snippets, plugins? If you open the developer tools (available from the view menu), does the console report any errors? If you inspect the element for that part of the pane, what does it report it as?
Don’t know if this is recommended, but going into the .obsidian folder (through the operating system’s file explorer), you can get to a file called appearance.json. Quitting Obsidian and then moving that file to a backup location (such as the desktop), forces Obsidian to recreate the appearance file when the vault is next opened, resetting the vault’s appearance back to the default settings. Could duplicate your vault and try that. You would need to reapply your appearance settings afterwards.
Also, what version of Obsidian are you using, and what installer version?
Angel