As far as I know all scaling in Construct Classic under DX9 uses ugly interpolation which makes everything blurry, so crisp retro pixel-art games can't be scaled up without being utterly ruined. Unless I haven't found an option for it that already exists, can we expect this in a later version of Construct Classic?
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Forgot to clarify that I meant when the entire viewport is resized via the "stretch window content" setting. I already know about point sampling.
You probably shouldn't count on any major changes to the renderer.
What you can do is draw the images scaled up, IE instead of 1 pixel use a 2x2 brush.
There are also a few fx's you could try.