blitz-shell: CursorMoved doesn't request_redraw, so DOM-driven drags lag behind the cursor

[postadelmaga]

In blitz-shell/src/window.rs, the WindowEvent::CursorMoved handler forwards the move to the DOM but — unlike every other event handler in the file — never calls self.request_redraw():

WindowEvent::CursorMoved { position, .. } => {
    let LogicalPosition::<f32> { x, y } = position.to_logical(self.window.scale_factor());
    self.mouse_pos = (x, y);
    let event = UiEvent::MouseMove(BlitzMouseButtonEvent { x, y, /* … */ });
    self.doc.handle_ui_event(event);
    // no request_redraw() here
}

(observed in blitz-shell 0.2.3)

Impact

Any app that implements dragging purely in the DOM — onmousedown starts a drag, onmousemove updates a signal, the element re-renders at the new position — sees the dragged element trail behind the cursor. Because the move doesn't request a repaint, the new frame only shows up on the next event that does request one (mouse-up, wheel, etc.), so the element catches up only when you pause or release. Same effect for DOM-driven panning.

Expectation

A mouse-move that changes the DOM should repaint promptly, like in a browser.

Suggested fix

Keeps idle hover cheap by only repainting while a button is held (self.buttons is already in scope):

self.doc.handle_ui_event(event);
if !self.buttons.is_empty() {
    self.request_redraw();
}

Alternatively, unconditionally request_redraw() on CursorMoved if the idle-hover cost is acceptable. Happy to open a PR.

[nicoburns]

The optimisation to only repaint when it is actually neded is important, so we'll want to try and keep that. I also think we can do better than "when mouse button is pressed", which may lead to both false positives and false negatives. If I am understanding things correctly, the thing which actually causes a redraw to be required is the DOM being mutated. So I would suggest:

• Adding a has_mutated (or similarly named) field to DocumentMutator
• Default this field to false, but set it to true when any kind of mutation is performed.
• In the Drop implementation of DocumentMutator, call request_redraw (using the BaseDocument's ShellProvider if the field is true).