Let's break down what's happening here and how to fix this frustrating issue:

The Root Cause #

When your Activity first launches, the onPageSelected callback from ViewPager2 fires midway through the initial layout measurement cycle. At this point, your TextView has already been measured with its default empty state (remember, tools:text only works for preview and doesn't affect runtime). When you set the text programmatically at this stage, the TextView doesn't trigger a full re-measurement of its parent ConstraintLayout (and ScrollView) — so it gets stuck at the initial single-line height it was assigned during the first pass.

Once you swipe the ViewPager2, the page change triggers a full layout invalidation across the entire view hierarchy. This forces the TextView to re-measure itself with the actual text content, which is why it finally adjusts to the correct height. The weird textSize quirk? Without an explicit text size, the default size calculation gets even more confused during that initial measurement pass, making the problem worse.

Solutions to Fix the Initial Height Issue #

1. Manually Set Initial Text + Trigger Re-Layout

Instead of relying solely on the onPageSelected callback for the first page, set the text explicitly after initializing your ViewPager2, then force a re-layout to propagate the change:

// After setting up your ViewPager2 adapter and data list
val initialData = dataList.first()
descriptionTextView.text = initialData.description

// Trigger a full re-measurement of the TextView and its parent constraint layout
descriptionTextView.requestLayout()
constraintLayout.requestLayout()

This ensures the text is set before the final layout pass, or at least forces the view hierarchy to recalculate heights immediately after setting the text.

2. Post the Text Update to the TextView's Message Queue

If you want to keep using the onPageSelected callback for all pages (including the first), wrap the text update in a post call. This delays the text change until after the current layout cycle completes, giving the system time to properly measure the new content:

viewPager2.registerOnPageChangeCallback(
    object : ViewPager2.OnPageChangeCallback() {
        override fun onPageSelected(position: Int) {
            super.onPageSelected(position)
            val currentData = dataList[position]
            
            descriptionTextView.post {
                descriptionTextView.text = currentData.description
                // Optional: Force re-layout if the post alone doesn't work
                descriptionTextView.requestLayout()
            }
        }
    }
)

3. Clean Up Redundant Attributes

Your TextView has android:layout_gravity="center" which is redundant in a ConstraintLayout (constraints already handle positioning). Removing it won't fix the core issue, but it cleans up the layout and avoids potential measurement conflicts:

<TextView
    android:id="@+id/descriptionTextView"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    app:layout_constraintTop_toBottomOf="@id/viewPager2"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintBottom_toBottomOf="parent"
    tools:text="text that is way too long to fit into one line and thereby has to have two lines"
    android:background="#0A0"
    android:textColor="@color/primaryTextColor"
    android:textSize="18sp"
    android:gravity="center" />

Why Your Previous Fixes Didn't Work #

• forceLayout(): This only marks the view as needing layout, but doesn't trigger an immediate layout pass or propagate the change up to parent views. You need requestLayout() for that.
• scrollHorizontally="false": This prevents horizontal scrolling but doesn't affect vertical measurement logic.
• lines="3"/maxLines="6": These fix or cap the height to a static number, but don't solve the dynamic measurement issue on initial load.

内容的提问来源于stack exchange，提问作者Benjamin Basmaci