fix: prevent horizontal scrolling in the message viewport

feature/mail/message/reader/impl/src/androidHostTest/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProviderTest.kt

@@ -0,0 +1,80 @@
+package net.thunderbird.feature.mail.message.reader.impl.css
+
+import assertk.assertThat
+import assertk.assertions.contains
+import kotlin.test.Test
+import net.thunderbird.core.common.mail.html.HtmlSettings
+import net.thunderbird.feature.mail.message.reader.api.css.CssClassNameProvider
+import net.thunderbird.feature.mail.message.reader.api.css.CssVariableNameProvider
+
+class DefaultGlobalCssStyleProviderTest {
+
+    @Test
+    fun `style should set main content box sizing to border box`() {
+        // Arrange
+        val testSubject = createTestSubject()
+
+        // Act
+        val style = testSubject.create(createHtmlSettings()).style
+
+        // Assert
+        assertThat(style).contains("box-sizing: border-box")
+    }
+
+    @Test
+    fun `style should preserve main content width and padding`() {
+        // Arrange
+        val testSubject = createTestSubject()
+
+        // Act
+        val style = testSubject.create(createHtmlSettings()).style
+        val mainContentRule = style.mainContentRule()
+
+        // Assert
+        assertThat(mainContentRule).contains("width: 100%")
+        assertThat(mainContentRule).contains("padding: 0 8px")
+    }
+
+    @Test
+    fun `style should preserve pre wrapping`() {
+        // Arrange
+        val testSubject = createTestSubject()
+
+        // Act
+        val style = testSubject.create(createHtmlSettings()).style
+
+        // Assert
+        assertThat(style).contains("white-space: pre-wrap")
+    }
+
+    private fun createTestSubject(): DefaultGlobalCssStyleProvider.Factory {
+        return DefaultGlobalCssStyleProvider.Factory(
+            cssClassNameProvider = FakeCssClassNameProvider,
+            cssVariableNameProvider = FakeCssVariableNameProvider,
+        )
+    }
+
+    private fun createHtmlSettings(): HtmlSettings {
+        return HtmlSettings(
+            useDarkMode = false,
+            useFixedWidthFont = false,
+        )
+    }
+
+    private fun String.mainContentRule(): String {
+        return substringAfter("  .root.main-content {")
+            .substringBefore("  .root.main-content pre {")
+    }
+
+    private object FakeCssClassNameProvider : CssClassNameProvider {
+        override val defaultNamespaceClassName = "default-namespace"
+        override val rootClassName = "root"
+        override val mainContentClassName = "main-content"
+        override val plainTextMessagePreClassName = "plain-text-message-pre"
+        override val signatureClassName = "signature"
+    }
+
+    private object FakeCssVariableNameProvider : CssVariableNameProvider {
+        override val blockquoteDefaultBorderLeftColor = "--blockquote-default-border-left-color"
+    }
+}

feature/mail/message/reader/impl/src/commonMain/kotlin/net/thunderbird/feature/mail/message/reader/impl/css/DefaultGlobalCssStyleProvider.kt

@@ -26,6 +26,7 @@ internal class DefaultGlobalCssStyleProvider private constructor(
         |    -webkit-user-select: auto;
         |  }
         |  .${cssClassNameProvider.rootClassName}.${cssClassNameProvider.mainContentClassName} {
+        |    box-sizing: border-box;
         |    width: 100%;
         |    overflow-wrap: break-word;
         |    padding: 0 8px;