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Fix a pause in the event loop when clicking the title bar on windows (#4136)

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* Fix a pause in the event loop when clicking the title bar on windows

When clicking the title bar on Windows, to drag the window, there is
a noticible pause in continuous redraw requests. This was fixed
in #839 and then regressed in #1852. The cursor blinks in both
cases and is unrelated. The regression made the blink happen after
the pause instead of immediately.

* Update the event loop pause note on the WM_NCLBUTTONDOWN handler

The application example was also updated to optionally animate the fill color
in order to demonstrate continuous redraw without pauses in the event
loop.
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aloucks 2025-03-16 21:27:27 -04:00 • committed by GitHub
parent ae28eea406
commit 2b4e8ef916
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25
src/platform_impl/windows/event_loop.rs
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@ -1135,6 +1135,31 @@ unsafe fn public_window_callback_inner(
WM_NCLBUTTONDOWN => {
if wparam == HTCAPTION as _ {
// Prevent the user event loop from pausing when left clicking the title bar.
//
// When the user interacts with the title bar, Windows enters the modal event
// loop. Currently, a left click causes a pause for about 500ms. Sending a dummy
// mouse-move event seems to cancel the modal loop early, preventing the pause.
// The application will never see this dummy event.
//
// The mouse coordinates are encoded into the lparam value, however the WM_MOUSEMOVE
// event is not using the same coordinate system of the WM_NCLBUTTONDOWN event.
// One uses client-area coordinates and the other is screen-coordinates. In any
// case, passing the lparam as-is with the dummy event does not seem the cancel
// the modal loop.
//
// However, passing in a value of 0 has been observed to always cancel the pause.
//
// Other notes:
//
// For some unknown reason, the cursor will blink when clicking the title bar.
// Cancelling the modal loop early causes the blink to happen *immediately*.
// Otherwise, the blank happens *after* the pause.
//
// When right-click the title bar, the system window menu is presented to the user,
// and the modal event loop begins. This dummy event does *not* prevent the freeze
// in the main event loop caused by that popup menu.
let lparam = 0;
unsafe { PostMessageW(window, WM_MOUSEMOVE, 0, lparam) };
}
result = ProcResult::DefWindowProc(wparam);