Always honor scroll intent on mousewheel

This commit introduces a change in the way we respond to mousewheel
events. In particular, if `delta * scrollSensitivity` is less than 1 or
greater than -1, we will round it up or down in an attempt to honor the
user's scroll intent.

spec/text-editor-component-spec.js

@@ -1209,7 +1209,6 @@ describe('TextEditorComponent', () => {
       }
 
       {
-        global.debug = true
         const expectedScrollTop = component.getScrollTop()
         const expectedScrollLeft = 20 * (scrollSensitivity / 100)
         component.didMouseWheel({deltaX: 20, deltaY: -10})
@@ -1228,6 +1227,40 @@ describe('TextEditorComponent', () => {
       }
     })
 
+    it('always scrolls by a minimum of 1, even when the delta is small or the scroll sensitivity is low', () => {
+      const scrollSensitivity = 10
+      const {component, editor} = buildComponent({height: 50, width: 50, scrollSensitivity})
+
+      {
+        component.didMouseWheel({deltaX: 0, deltaY: 3})
+        expect(component.getScrollTop()).toBe(1)
+        expect(component.getScrollLeft()).toBe(0)
+        expect(component.refs.content.style.transform).toBe(`translate(0px, -1px)`)
+      }
+
+      {
+        component.didMouseWheel({deltaX: 4, deltaY: 0})
+        expect(component.getScrollTop()).toBe(1)
+        expect(component.getScrollLeft()).toBe(1)
+        expect(component.refs.content.style.transform).toBe(`translate(-1px, -1px)`)
+      }
+
+      editor.update({scrollSensitivity: 100})
+      {
+        component.didMouseWheel({deltaX: 0, deltaY: -0.3})
+        expect(component.getScrollTop()).toBe(0)
+        expect(component.getScrollLeft()).toBe(1)
+        expect(component.refs.content.style.transform).toBe(`translate(-1px, 0px)`)
+      }
+
+      {
+        component.didMouseWheel({deltaX: -0.1, deltaY: 0})
+        expect(component.getScrollTop()).toBe(0)
+        expect(component.getScrollLeft()).toBe(0)
+        expect(component.refs.content.style.transform).toBe(`translate(0px, 0px)`)
+      }
+    })
+
     it('inverts deltaX and deltaY when holding shift on Windows and Linux', async () => {
       const scrollSensitivity = 50
       const {component, editor} = buildComponent({height: 50, width: 50, scrollSensitivity})

src/text-editor-component.js

@@ -1511,11 +1511,15 @@ class TextEditorComponent {
     let {deltaX, deltaY} = event
 
     if (Math.abs(deltaX) > Math.abs(deltaY)) {
-      deltaX = deltaX * scrollSensitivity
+      deltaX = (Math.sign(deltaX) === 1)
+        ? Math.max(1, deltaX * scrollSensitivity)
+        : Math.min(-1, deltaX * scrollSensitivity)
       deltaY = 0
     } else {
       deltaX = 0
-      deltaY = deltaY * scrollSensitivity
+      deltaY = (Math.sign(deltaY) === 1)
+        ? Math.max(1, deltaY * scrollSensitivity)
+        : Math.min(-1, deltaY * scrollSensitivity)
     }
 
     if (this.getPlatform() !== 'darwin' && event.shiftKey) {