Separate scrolling from clicking

[lhecker]

Closes #410
Closes #588

[MarkusBillharz]

@lhecker

#588 (comment)

But also noticed that it was still not possible to start a selection by:
Left->Scroll while holding the mouse still.

It seems this edge case slipped through here. Currently, the selection starts from where the mouse is first moved, rather than where the button-down was initiated.

Edit: In case it is of interest, I rebased my branch just to fix it locally, still seems sound to me.

I still feel like it might be of some value in terms of reasoning,
but since I’m not sure what you disliked about the initial PR, I’ve held off on creating a new one.

Cheers~

[lhecker]

I don't doubt that your diff solves the issue but I'm trying to fit it into a long term solution where scrolling isn't the same as clicking in the first place. Dragging should also not be a boolean member like that, but rather the UI system should have distinct mouse-down/up events. Basically, I'm looking for a holistic solution rather than an immediate fix.

[lhecker]

I've filed #608 to track this.