This bug is still present in version 0.90. The {width,height}_request in gtk/gtkbuilder/dialog_conffile.ui should be moved from the GtkTextView "textview_diff" (showing the diff) up to the GtkScrolledWindow "scrolledwindow1" that contains it. Also, the whole GtkDialog should specify a default-width otherwise the natural width of the "label_message" (cointaining one ver long line of text) will dominate the dialog's width.
Patch attached. Cheers, Roderich
--- gtk/gtkbuilder/dialog_conffile.ui.orig 2020-01-25 14:15:30.000000000 +0100 +++ gtk/gtkbuilder/dialog_conffile.ui 2020-02-28 14:18:34.379963497 +0100 @@ -3,6 +3,7 @@ <interface> <requires lib="gtk+" version="3.0"/> <object class="GtkDialog" id="dialog_conffile"> + <property name="default-width">600</property> <property name="visible">False</property> <property name="can_focus">False</property> <property name="border_width">5</property> @@ -128,10 +129,10 @@ <property name="visible">True</property> <property name="can_focus">True</property> <property name="shadow_type">in</property> - <child> - <object class="GtkTextView" id="textview_diff"> <property name="width_request">600</property> <property name="height_request">400</property> + <child> + <object class="GtkTextView" id="textview_diff"> <property name="visible">True</property> <property name="can_focus">True</property> <property name="editable">False</property>