SynthDiff

Explorer can't tell you how two folders differ. SynthDiff walks both trees in parallel and pairs entries by name, case-insensitively, as Windows does. Every item is marked Same, Different, Only left, Only right or File vs folder, and each folder shows how many differences it contains.

There are two ways to compare: size + date is fast and allows 2 seconds of timestamp drift, so copies on USB sticks don't all look changed; content compares same-size files byte by byte. Filter chips with counts show or hide each kind of difference, and an ignore list such as {.git, node_modules, *.tmp} skips noise on both sides.

You can act straight from the list, on one item or many: copy to the other side, cut, copy, delete, open in Explorer or show Properties. Copy and delete go through the Windows shell, with its progress dialog, the Recycle Bin and Ctrl+Z undo. SynthDiff never compares a folder with itself, never follows symlinks or junctions, and confirms every action first.