I just noticed what may be a bug (I don't see a bug report on it yet, though). The dissolve blend mode doesn't appear to work consistently. In one case the application of a Dissolve blend mode produces the spotty effect you'd expect with 50% opacity. In other cases, it just produces the effect you'd expect with the opacity at 50% under Normal blend mode.

For example:

1. Background layer (no alpha channel): Make a square selection. Do a bucket fill with Bucket Tool options opacity set to 50% and Blend mode set to Dissolve. Result: spotty bucket fill (as expected).

2. Background layer (no alpha channel): No selection. Do a bucket fill with Bucket Tool options opacity set to 50% and Blend mode set to Dissolve. Result: smooth fill that is partially transparent (ie it's not spotty).

3. 2nd layer (with alpha channel): Make a square selection. Do a bucket fill with Bucket Tool options opacity set to 50% and Blend mode set to Dissolve. Result: smooth fill that is partially transparent (ie it's not spotty).

4. 2nd layer (with alpha channel): No selection. Do a bucket fill with Bucket Tool options opacity set to 50% and Blend mode set to Dissolve. Result: smooth fill that is partially transparent (ie it's not spotty).

Additionally, the layer blend mode, when set to dissolve, doesn't seem to ever produce the expected spotty effect. It appears that dissolve only works when applied with painting tools within a selection and on a layer with no alpha channel.

Is this a known problem? Am I just using it incorrectly? I can write a bug report for it if its new. I just wanted to check first to be sure. I'm using 1.2.3 and have not tested this against the 1.3.x branch.