Moving last markup element via drag&drop removes markup icon in board position cell

[herzbube]

1. Navigate to a board position that contains exactly one markp element of type symbol, marker or label.
2. Start a drag&drop (=panning) gesture.
3. Drag the markup element to a different intersection but don't drop it yet.

Expected: The markup icon in the current board position cell remains visible. Actual: The markup icon in the current board position cell disappears as soon as the markup element is dragged to a different intersection. It does not reappear while the gesture is ongoing. The icon reappears only when the gesture ends.

[herzbube]

Currently this is treated as a "known bug" with very low priority, because the icon disappears only temporarily, and the disappearance is hardly noticeable. Furthermore, the amount of work needed to fix this issue vs. the severity of the bug seems seriously disproportionate.

[herzbube]

Preliminary analysis

• The root cause of the issue is that when the gesture begins the markup element is actually removed from the model data (GoNodeMarkup), causing all the notifications to be sent as if the removal had been permanent. The board position cell updates itself accordingly, not being aware that the removal was only temporary.
• The temporary removal from the model data was the most elegant solution for the exceedingly complicated drawing issues that popped up in SymbolsLayerDelegateand LabelsLayerDelegate when the drag&drop gesture was implemented. A solution for this bug should therefore probably not touch this temporary removal, but instead go in the direction of having a special notification mechanism that triggers only drawing, but keeps the rest of the application unaware of the temporary removal.

Some ideas

• Notify drawing layers of the temporary removal via BoardView instead of via global notification markupOnPointsDidChange.
  • Assuming this is implemented: When the gesture completes it executes HandleMarkupEditingInteractionCommand which in turn sends markupOnPointsDidChange.
  • This means that clients of markupOnPointsDidChange would see a new markup element appearing at the end of the gesture, but they would not see the markup element being removed because they never received a markupOnPointsDidChange.
  • Consequence: markupOnPointsDidChange would need to be improved so that it can express a move operation (instead of just a remove/place operation).
• Don't do temporary removal, instead implement some kind of "filtering" in the drawing layers. Gesture handling notifies the layers via something like BLVDEvent...Filter.
  • If the solution goes in this direction then at the gesture end two markupOnPointsDidChange would need to be sent: One for removing the markup element, and one for placing it.
  • Alternatively, markupOnPointsDidChange would need to be improved so that it can express a move operation (instead of just a remove/place operation).