Fix jumpy dragging when snapDuration !== slotDuration

[jfrancos]

Hi! 👋

Firstly, thanks for your work on this project! 🙂

Today I used patch-package to patch @fullcalendar/timegrid@7.0.0-beta.4 for the project I'm working on.

with

snapDuration={{ minutes: 15 }}
slotDuration={{ minutes: 30 }}

Before: dragging an event up or down is jumpy (up 30 minutes, down 15 minutes, etc) and mouse gets out of sync:

Screen.Recording.2025-01-25.at.1.08.48.PM.mov

After: smooth and mouse in sync:

Screen.Recording.2025-01-25.at.1.01.30.PM.mov

Here is the diff that solved my problem:

diff --git a/node_modules/@fullcalendar/timegrid/internal.js b/node_modules/@fullcalendar/timegrid/internal.js
index ac5854a..b7cf5d8 100644
--- a/node_modules/@fullcalendar/timegrid/internal.js
+++ b/node_modules/@fullcalendar/timegrid/internal.js
@@ -684,7 +684,7 @@ class TimeGridCols extends DateComponent {
         const slatTop = slatIndex * slatHeight;
         const partial = (positionTop - slatTop) / slatHeight; // floating point number between 0 and 1
         const localSnapIndex = Math.floor(partial * snapsPerSlot); // the snap # relative to start of slat
-        const snapIndex = slatIndex + localSnapIndex * snapsPerSlot;
+        const snapIndex = slatIndex * snapsPerSlot + localSnapIndex;
         const time = addDurations(dateProfile.slotMinTime, multiplyDuration(snapDuration, snapIndex));
         const start = dateEnv.add(cell.date, time);
         const end = dateEnv.add(start, snapDuration);

Maybe this is helpful for fullcalendar/fullcalendar#4865? Or maybe it breaks something. Haven't tested beyond that it seems to fix my own issue.

Thank you for your hard work!

This issue body was partially generated by patch-package.

[jfrancos]

fullcalendar/fullcalendar#7874

[jfrancos]

@acerix @arshaw Can you give some feedback on this fix for fullcalendar/fullcalendar#4865?

Happy to open a PR in https://github.com/fullcalendar/fullcalendar/pulls too, if that's the right thing to do.