SDL: emscripten: Decrease vertical scroll speed by using deltaMode

From 94c1276a5ff347eb9ce3ae7778f1192db2790027 Mon Sep 17 00:00:00 2001
From: Thomas Ballinger <[EMAIL REDACTED]>
Date: Sun, 7 Nov 2021 20:49:32 -0500
Subject: [PATCH] emscripten: Decrease vertical scroll speed by using deltaMode

Reference issue #4623.
---
 src/video/emscripten/SDL_emscriptenevents.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/video/emscripten/SDL_emscriptenevents.c b/src/video/emscripten/SDL_emscriptenevents.c
index 0c1b94c710..bae18d2128 100644
--- a/src/video/emscripten/SDL_emscriptenevents.c
+++ b/src/video/emscripten/SDL_emscriptenevents.c
@@ -409,7 +409,22 @@ static EM_BOOL
 Emscripten_HandleWheel(int eventType, const EmscriptenWheelEvent *wheelEvent, void *userData)
 {
     SDL_WindowData *window_data = userData;
-    SDL_SendMouseWheel(window_data->window, 0, (float)wheelEvent->deltaX, (float)-wheelEvent->deltaY, SDL_MOUSEWHEEL_NORMAL);
+
+    float deltaY = wheelEvent->deltaY;
+
+    switch (wheelEvent->deltaMode) {
+        case DOM_DELTA_PIXEL:
+            deltaY /= 100; /* 100 pixels make up a step */
+            break;
+        case DOM_DELTA_LINE:
+            deltaY /= 3; /* 3 lines make up a step */
+            break;
+        case DOM_DELTA_PAGE:
+            deltaY *= 80; /* A page makes up 80 steps */
+            break;
+    }
+
+    SDL_SendMouseWheel(window_data->window, 0, (float)wheelEvent->deltaX, -deltaY, SDL_MOUSEWHEEL_NORMAL);
     return SDL_GetEventState(SDL_MOUSEWHEEL) == SDL_ENABLE;
 }