The Mirror app has Zoom In and Zoom Out as scaling options, but the playdate screen window will always introduce a few thick pixel lines due to it not matching with integer multiples of playdate's native screen resolution.
Implementing Integer Scaling would ensure that all pixels will be even (1x for 400x240, 2x for 800x480, and so on...).
Also consider adding bilinear interpolation for lines that are thicker on non-integer scaled resolution.
Note that this doesn't affect footage recorded through the app's Capture feature.
What resolution and scale are you running your monitor at? It's supposed to do full 1x and 2x scales but something isn't quite right; the combination of any number of scales/resolution on Windows makes it a bit tricky to handle.
I'm currently running the latest stable Windows 10 64-bit build with the resolution of 5120x1440, which is spread across two 2560x1440 monitors.
The image I posted is the one with the furthest zoom possible.
What is your scale % in the Display settings? Thanks!
Under the enabled advanced scaling settings in the Display settings, I let Windows attempting to correct certain apps from being displayed blurry and set the ratio at 125%.
Just wanted to let you know I have this fixed for the next release. Thanks again for the report!