Eeschema, importFile: build the HTML report messages before displaying the dialog.

Fixes #8507
https://gitlab.com/kicad/code/kicad/issues/8507

common/dialogs/wx_html_report_box.h

@@ -43,9 +43,21 @@ public:
     void SetUnits( EDA_UNITS aUnits ) { m_units = aUnits; }
     EDA_UNITS GetUnits() const override { return m_units; }
 
+    /**
+     * In immediate mode, messages are flushed as they are added.
+     * Required for progress-related reports, but can be very slow for larger reports.
+     */
     void SetImmediateMode() { m_immediateMode = true; }
 
+    /**
+     * Build the HTML messages page.
+     * Call it if the immediate mode is not actived to be able to display them
+     */
     void Flush();
+
+    /**
+     * Delete the stored messages
+     */
     void Clear();
 
 private:

eeschema/files-io.cpp

@@ -1118,7 +1118,10 @@ bool SCH_EDIT_FRAME::importFile( const wxString& aFileName, int aFileType )
             Schematic().SetRoot( pi->Load( aFileName, &Schematic() ) );
 
             if( reporter->m_Reporter->HasMessage() )
+            {
+                reporter->m_Reporter->Flush();  // Build HTML messages
                 reporter->ShowModal();
+            }
 
             pi->SetReporter( &WXLOG_REPORTER::GetInstance() );
             delete reporter;