In the advanced digitizing toolbar, QGIS has these 3 tools: Split Features, Split Parts, and Merge Features. Is there a tool or equivalent algorithm for the logical fourth one: "Merge Parts"?
This would have the opposite effect of Split Parts, which takes one part of a multipart feature and splits it into two adjoining parts of the same feature. Conversely, "Merge Parts" would take two (or more) adjoining parts of the same feature and make them a single part of that feature. It would not affect the singlepart/multipart status of the feature (or the layer as a whole).
In the image below there are two multipolygon features, which I realize I should have called Feature A and Feature B (rather than Polygon A and Polygon B). Feature A has 2 parts, and Feature B has 4 parts; note that parts 1 and 2 in Feature B share a common boundary. I've selected Feature B, and I'd like to execute a tool to join parts 1 and 2 into a single part, as shown in the "after" section. Feature B would then still be a multipolygon, but would have only 3 parts instead of 4.
The closest I can find is the "merge lines" algorithm; however this would not work with multipolygon features. It also merges all connected parts of a line feature, instead of just selected ones. (Is it even possible to select individual parts?) Is there anything else I'm missing?