Themes can set separate ActiveTabBackgroundColor and InactiveTabBackgroundColor values (#5959).
Thumbnails in the command palette
Type & in the command palette to open a thumbnail grid of the current document, with the current page selected. Page numbers appear as themed badges on the thumbnails.
Snap line annotations to 45 degrees
Hold Shift while drawing a line annotation, or dragging a line’s endpoint, to snap it to multiples of 45 degrees (#6125).
Smaller UX improvements
Annotation tooltips no longer show the author unless ShowAnnotationAuthorInTooltip is on.
Selecting several files in Explorer and choosing Open opens them all in one SumatraPDF instance.
Page Up/Down turn pages while you type a range in the Extract / Delete Pages dialogs (#6116).
On the home page, the Command Palette and Keyboard Shortcuts buttons show their shortcut keys in tooltips.
Show in folder uses SumatraPDF’s own file picker when Use SumatraPDF file picker is on.