Don't throw an error when setting an incredibly small lineHeight

Instead, if the measured line height equals 0, default it to 1 so that
the editor component doesn't start computing `NaN` or `Infinity` values
due to e.g. dividing by 0.

We should probably consider sanitizing line heights smaller than a
certain threshold, but that's non trivial because line height is
expressed as a multiplier of the font size. Also, users may style the
`line-height` property via CSS, which may still throw errors when using
small values.

spec/text-editor-component-spec.js

@@ -119,6 +119,44 @@ describe('TextEditorComponent', () => {
       }
     })
 
+    it('re-renders lines when their height changes', async () => {
+      const {component, element, editor} = buildComponent({rowsPerTile: 3, autoHeight: false})
+      element.style.height = 4 * component.measurements.lineHeight + 'px'
+      await component.getNextUpdatePromise()
+      expect(element.querySelectorAll('.line-number:not(.dummy)').length).toBe(9)
+      expect(element.querySelectorAll('.line:not(.dummy)').length).toBe(9)
+
+      element.style.lineHeight = '2.0'
+      TextEditor.didUpdateStyles()
+      await component.getNextUpdatePromise()
+      expect(element.querySelectorAll('.line-number:not(.dummy)').length).toBe(6)
+      expect(element.querySelectorAll('.line:not(.dummy)').length).toBe(6)
+
+      element.style.lineHeight = '0.7'
+      TextEditor.didUpdateStyles()
+      await component.getNextUpdatePromise()
+      expect(element.querySelectorAll('.line-number:not(.dummy)').length).toBe(12)
+      expect(element.querySelectorAll('.line:not(.dummy)').length).toBe(12)
+
+      element.style.lineHeight = '0.05'
+      TextEditor.didUpdateStyles()
+      await component.getNextUpdatePromise()
+      expect(element.querySelectorAll('.line-number:not(.dummy)').length).toBe(13)
+      expect(element.querySelectorAll('.line:not(.dummy)').length).toBe(13)
+
+      element.style.lineHeight = '0'
+      TextEditor.didUpdateStyles()
+      await component.getNextUpdatePromise()
+      expect(element.querySelectorAll('.line-number:not(.dummy)').length).toBe(13)
+      expect(element.querySelectorAll('.line:not(.dummy)').length).toBe(13)
+
+      element.style.lineHeight = '1'
+      TextEditor.didUpdateStyles()
+      await component.getNextUpdatePromise()
+      expect(element.querySelectorAll('.line-number:not(.dummy)').length).toBe(9)
+      expect(element.querySelectorAll('.line:not(.dummy)').length).toBe(9)
+    })
+
     it('makes the content at least as tall as the scroll container client height', async () => {
       const {component, element, editor} = buildComponent({text: 'a', height: 100})
       expect(component.refs.content.offsetHeight).toBe(100)

src/text-editor-component.js

@@ -2046,7 +2046,7 @@ class TextEditorComponent {
   }
 
   measureCharacterDimensions () {
-    this.measurements.lineHeight = this.refs.characterMeasurementLine.getBoundingClientRect().height
+    this.measurements.lineHeight = Math.max(1, this.refs.characterMeasurementLine.getBoundingClientRect().height)
     this.measurements.baseCharacterWidth = this.refs.normalWidthCharacterSpan.getBoundingClientRect().width
     this.measurements.doubleWidthCharacterWidth = this.refs.doubleWidthCharacterSpan.getBoundingClientRect().width
     this.measurements.halfWidthCharacterWidth = this.refs.halfWidthCharacterSpan.getBoundingClientRect().width